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X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray emission from celestial objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and satellites. X-ray astronomy is part of space science.

X-ray emission is expected in sources which contain an extremely hot gas at temperatures from a million to hundred million degrees kelvin. In general, this occurs in objects where the atoms and/or electrons have a very high energy. The discovery of the first cosmic X-ray source in 1962 came as a surprise. This source is called Scorpius X-1, the first X-ray source found in the constellation Scorpius. Based on discoveries in this new field, Riccardo Giacconi received the Nobel Prize in Physics in 2002. It was found that the X-ray emission of Sco X-1 was 10,000 times greater than its optical emission, based on a precise location obtained with a modulation collimator - a specific type of coded aperture imager. In addition, the energy output in X-rays is 100,000 times greater than the total emission of the Sun in all wavelengths. It is now known that such X-ray sources are compact stars, such as neutron stars and black holes. The energy source is gravity. Gas is heated by the fall in the strong gravitational field of celestial objects.

Many thousands of X-ray sources are known. In addition, it appears that the space between galaxies in a cluster of galaxies is filled with a very hot, but very dilute gas at a temperature between 10 and 100 megakelvin (MK). The total amount of hot gas is five to ten times the total mass in the visible galaxies.

Featurette

GOES 14 was launched into orbit on Jun 27 2009 at 22:51 GMT from Space Launch Complex 37B at the Cape Canaveral Air Force Station. GOES 14 is the most recent satellite to be launched with X-ray detection capability. The importance of X-ray astronomy is exemplified in the use of an X-ray imager such as the one on GOES 14 for the early detection of solar flares, CMEs and other X-ray generating phenomena that impact the Earth.

Detection and imaging of X-rays

X-rays span 3 decades in wavelength ~(8 nm - 8 pm), frequency ~(50 PHz - 50 EHz) and energy ~(0.12 - 120 keV). In terms of temperature, 1 eV = 11,604 K. X-rays (0.12 to 120 keV) correspond to 1.39 x 10 (1.39 MK) to 1.39 x 10 K (1.39 GK). From 10 to 0.1 nanometers (nm) (about 0.12 to 12 keV) they are classified as soft X-rays, and from 0.1 nm to 0.01 nm (about 12 to 120 keV) as hard X-rays.

Closer to the visible range of the electromagnetic spectrum is the ultraviolet. The draft ISO standard on determining solar irradiances (ISO-DIS-21348) describes the ultraviolet as ranging from ~10 nm to ~400 nm. That portion closest to X-rays is often referred to as the "extreme ultraviolet" (EUV or XUV). When an EUV photon is absorbed, photoelectrons and secondary electrons are generated by ionization, much like what happens when X-rays or electron beams are absorbed by matter.

The distinction between X-rays and gamma rays has changed in recent decades. Originally, the electromagnetic radiation emitted by X-ray tubes had a longer wavelength than the radiation emitted by radioactive nuclei (gamma rays). So older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10 m, defined as gamma rays. However, as shorter wavelength continuous spectrum "X-ray" sources such as linear accelerators and longer wavelength "gamma ray" emitters were discovered, the wavelength bands largely overlapped. The two types of radiation are now usually distinguished by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.

Although the more energetic X-rays, photons with an energy greater than 30 keV (4,800 aJ), can penetrate the air at least for distances of a few meters, the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface (they would have been detected and medical X-ray machines would not work if this was not the case). X-rays in the 0.5 to 5 keV (80 to 800 aJ) range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; ninety percent of the photons in a beam of 3 keV (480 aJ) X-rays are absorbed by traveling through just 10 cm of air.

To detect X-rays from the sky, X-ray detectors must be flown above most of the Earth's atmosphere. There are three main methods of doing so: sounding rocket flights, balloons, and satellites.

Sounding rocket flights

A detector is placed in the nose cone section of a sounding rocket and launched above the atmosphere. This was first done at White Sands Missile Range in New Mexico with a V-2 rocket on Jan 28 1949 10:20 local time. X-rays from the Sun were detected by the USA Naval Research Laboratory Blossom experiment on board. An Aerobee 150 rocket launched on June 12 1962 detected the first X-rays from other celestial sources (Scorpius X-1). The largest drawback to rocket flights is their very short duration (just a few minutes above the atmosphere before the rocket falls back to Earth) and their limited field of view. A rocket launched from the United States will not be able to see sources in the southern sky; a rocket launched from Australia will not be able to see sources in the northern sky.

Normal Incidence X-ray Telescope (NIXT)

Using sounding rockets, the NIXT from the Harvard-Smithsonian Center for Astrophysics (CfA) has taken a unique set of high resolution full disk solar images. The telescope primary is 25 cm in diameter.

The solar corona consists of a low-density magnetized plasma at temperatures exceeding 10 K. The primary coronal emission is in the UV and soft X-ray range. The close connection between solar magnetic fields and the physical parameters of the corona implies a fundamental role for the magnetic field in coronal structuring and dynamics. Variability of the corona occurs on all temporal and spatial scales - at one extreme, as the result of plasma instabilities, and at the other extreme driven by the global magnetic flux emergence patterns of the solar cycle.

The telescope has flown on a Terrier/Black Brant vehicle. The primary reason for using multilayer coatings at XUV and soft X-ray wavelengths is because no single surface layer coating can provide acceptable X-ray reflectivity at wavelengths shorter than approx. 300 Å (30.0 nm) when used at normal incidence. For instance, at 173 Å (17.3 nm) the best materials have R ~0.001. By precise deposition of 50 alternating layers of Mo and Si, mirrors with R ~50 have been produced. When normal incidence mirror designs are employed, the immediate advantage is greatly improved image quality. The NIXT telescope recorded the highest resolution solar corona photographs in x-ray ever taken on its last three flights (1989-1991).

Balloons

Balloon flights can carry instruments to altitudes of up to 40 kilometers above sea level, where they are above as much as 99.997% of the Earth's atmosphere. Unlike a rocket where data are collected during a brief few minutes, balloons are able to stay aloft for much longer. However, even at such altitudes, much of the X-ray spectrum is still absorbed. X-rays with energies less than 35 keV (5,600 aJ) cannot reach balloons. On Jul 21 1964 the Crab Nebula supernova remnant is discovered to be a hard X-ray (15 - 60 keV) source by a scintillation counter flown on a balloon launched from Palestine, Texas, USA. This was likely the first balloon-based detection of X-rays from a discrete cosmic X-ray source.

High Resolution Gamma-ray and Hard X-ray Spectrometer (HIREGS)

One of the recent balloon-borne experiments was called the High Resolution Gamma-ray and Hard X-ray Spectrometer (HIREGS). It was first launched from McMurdo Station, Antarctica in December 1991, when steady winds carried the balloon on a circumpolar flight lasting for about two weeks.

High Energy Focusing Telescope (HEFT)

The High Energy Focusing Telescope (HEFT) is a balloon-borne experiment to image astrophysical sources in the hard X-ray (20-100 keV) band. Its maiden flight took place in May 2005 from Fort Sumner, New Mexico, USA. The angular resolution of HEFT is ~1.5'. HEFT makes use of tungsten-silicon multilayer coatings to extend the reflectivity of nested grazing-incidence mirrors beyond 10 keV. HEFT has an energy resolution of 1.0 keV full width at half maximum at 60 keV. HEFT was launched for a 25-hour balloon flight in May 2005. The instrument performed within specification and observed Tau X-1, the Crab Nebula.

Rockoons

The rockoon (a portmanteau of rocket and balloon) was a solid fuel rocket that, rather than being immediately lit while on the ground, was first carried into the upper atmosphere by a gas-filled balloon. Then, once separated from the balloon at its maximum height, the rocket was automatically ignited. This achieved a higher altitude, since the rocket did not have to move through the lower, thicker air layers.

The original concept of "Rockoons" was developed by Cmdr. Lee Lewis, Cmdr. G. Halvorson, S. F. Singer, and James A. Van Allen during the Aerobee rocket firing cruise of the USS Norton Sound on Mar 1 1949.

From Jul 17 to Jul 27 1956 the USA Naval Research Laboratory (NRL) shipboard launched 8 Deacon rockoons for solar ultraviolet and X-ray observations at ~30° N ~121.6° W, southwest of San Clemente Island, apogee: 120 km.

Satellites

A detector is placed on a satellite which is then put into orbit well above the Earth's atmosphere. Unlike balloons, instruments on satellites are able to observe the full range of the X-ray spectrum. Unlike sounding rockets, they can collect data for as long as the instruments continue to operate. In one instance, the Vela 5B satellite, the X-ray detector remained functional for over ten years.

Active X-ray observatory satellites

Satellites in use today include the XMM-Newton observatory (low to mid energy X-rays 0.1-15 keV) and the INTEGRAL satellite (high energy X-rays 15-60 keV). Both were launched by the European Space Agency. NASA has launched the Rossi X-ray Timing Explorer (RXTE), and the Swift and Chandra observatories. One of the instruments on Swift is the Swift X-Ray Telescope (XRT).

The GOES 14 spacecraft carries on board a Solar X-ray Imager to monitor the Sun’s X-rays for the early detection of solar flares, coronal mass ejections, and other phenomena that impact the geospace environment. It was launched into orbit on Jun 27 2009 at 22:51 GMT from Space Launch Complex 37B at the Cape Canaveral Air Force Station.

On Jan 30 2009 the Russian Federal Space Agency successfully launched the Koronas-Foton which carries several experiments to detect X-rays, including the TESIS telescope/spectrometer FIAN with SphinX soft X-ray spectrophotometer.

The Italian Space Agency (ASI) gamma-ray observatory satellite Astro-rivelatore Gamma ad Imagini Leggero (AGILE) has on board the Super-AGILE 15-45 keV hard X-ray detector. It was launched on Apr 23 2007 by the Indian PSLV-C8.

A soft X-ray solar imaging telescope is on board the GOES-13 weather satellite launched using a Delta IV from Cape Canaveral LC37B on May 24 2006. But, there have been no GOES 13 SXI images since December 2006.

Although the Suzaku X-ray spectrometer (the first micro-calorimeter in space) failed on Aug 8 2005 after launch on Jul 10 2005, the X-ray Imaging Spectrometer (XIS) and Hard X-ray Detector (HXD) are apparently still functioning.

Past X-ray observatory satellites

Past observatories include SMART-1, which contained an X-ray telescope for mapping lunar X-ray fluorescence, ROSAT, the Einstein Observatory (the first fully imaging X-ray telescope), the ASCA observatory, EXOSAT, and BeppoSAX. Uhuru was the first satellite launched specifically for the purpose of X-ray astronomy. Copernicus which carried an X-ray detector built by University College London's Mullard Space Science Laboratory made extensive X-ray observations. ANS could measure X-ray photons in the energy range 2 to 30 keV. Ariel 5 was dedicated to observing the sky in the X-ray band. HEAO-1 scanned the X-ray sky over 0.2 keV - 10 MeV. Hakucho was Japan's first X-ray astronomy satellite.

Array of Low Energy X-ray Imaging Sensors (ALEXIS)

The Array of Low Energy X-ray Imaging Sensors (ALEXIS) featured curved mirrors whose multilayer coatings reflect and focus low-energy X-rays or extreme ultraviolet light the way optical telescopes focus visible light. The launch of ALEXIS was provided by the United States Air Force Space Test Program on a Pegasus Booster on April 25, 1993. The spacing of the molybdenum (Mo) and silicon (Si) layers on each telescope's mirror is the primary determinant of the telescope's photon energy response function. ALEXIS operated for 12 yr.

OSO-3

The third Orbiting Solar Observatory (OSO 3) was launched on Mar 8 1967 into a nearly circular orbit of mean altitude 550 km, inclined at 33° to the equatorial plane, deactivated on Jun 28 1968, followed by reentry on Apr 4 1982. Its XRT consisted of a continuously spinning wheel (1.7 s period) in which the hard X-ray experiment was mounted with a radial view. The XRT assembly was a single thin NaI(Tl) scintillation crystal plus phototube enclosed in a howitzer-shaped CsI(Tl) anti-coincidence shield. The energy resolution was 45% at 30 keV. The instrument operated from 7.7 to 210 keV with 6 channels. OSO-3 obtained extensive observations of solar flares, the diffuse component of cosmic X-rays, and the observation of a single flare episode from Scorpius X-1, the first observation of an extrasolar X-ray source by an observatory satellite. Among the extrasolar X-ray sources OSO 3 observed were UV Cet, YZ CMi, EV Lac, and AD Leo, yielding upper soft X-ray detection limits on flares from these sources.

ESRO 2B (Iris)

ESRO-2B (Iris) was the first successful ESRO satellite launch. Iris was launched on May 17 1968, had an elliptical orbit with (initially) apogee 1086 km, perigee 326 km, and inclination 97.2 degrees, with an orbital period of 98.9 minutes. The satellite carried seven instruments to detect high energy cosmic rays, determine the total flux of solar X-rays, and measure trapped radiation, Van Allen belt protons and cosmic ray protons. Of special significance for X-ray astronomy were two X-ray instruments: one designed to detect wavelengths 1-20 Å (0.1-2 nm) (consisting of proportional counters with varying window thickness) and one designed to detect wavelengths 44-60 Å (4.4-6.0 nm) (consisting of proportional counters with thin mylar windows).

Wavelength dispersive X-ray spectroscopy (WDS) is a method used to count the number of X-rays of a specific wavelength diffracted by a crystal. WDS only counts x-rays of a single wavelength or wavelength band. In order to interpret the data, the expected elemental wavelength peak locations need to be known. For the ESRO-2B WDS X-ray instruments, calculations of the expected solar spectrum had to be performed and were compared to peaks detected by rocket measurements.

X-ray telescopes/mirrors

X-ray telescopes (XRTs) have varying directionality or imaging ability based on glancing angle reflection rather than refraction or large deviation reflection . This limits them to much narrow fields of view than visible or UV telescopes. The mirrors can be made of ceramic or metal foil .

The first X-ray telescope in astronomy was used to observe the Sun. The first X-ray picture of the Sun was taken in 1963, by a rocket-borne telescope.

The utilization of X-ray mirrors for extrasolar X-ray astronomy simultaneously requires

• the ability to determine the location at the arrival of an X-ray photon in two dimensions and
• a reasonable detection efficiency.

Detectors

Astronomy X-ray detectors have been designed and configured primarily for energy and occasionally for wave-length detection using a variety of techniques usually limited to the technology of the time.

Proportional counters

A proportional counter is a type of gaseous ionization detector that counts particles of ionizing radiation and measures their energy. It works on the same principle as the Geiger-Müller counter, but uses a lower operating voltage. All X-ray proportional counters consist of a windowed gas cell. Often this cell is subdivided into a number of low- and high-electric field regions by some arrangement of electrodes.

An individual medium energy proportional counter on EXOSAT had a front window of beryllium with aluminized kapton foil for thermal protection, a front chamber filled with an argon/CO₂ mixture, a rear chamber with xenon/CO₂, and a beryllium window separating the two chambers. The argon portion of the detector was optimized for 2-6 keV and the total energy ranges for both detectors was 1.5-15 keV and 5-50 keV, respectively.

The US portion of the Apollo-Soyuz mission (Jul 1975) carried a proportional counter system sensitive to 0.18-0.28 and 0.6-10.0 keV X-rays. The total effective area was 0.1 m, and there was a 4.5° FWHM circular FOV.

The French TOURNESOL instrument consisted of four proportional counters and two optical detectors. The proportional counters detected photons between 2 keV and 20 MeV in a 6° x 6° FOV. The visible detectors had a field of view of 5° x 5°. The instrument was designed to look for optical counterparts of high-energy burst sources, as well as performing spectral analysis of the high-energy events.

X-ray monitor

Monitoring generally means to be aware of the state of a system. A device that displays or sends a signal for displaying X-ray output from an X-ray generating source so as to be aware of the state of the source is referred to as an X-ray monitor in space applications. On Apollo 15 in orbit above the Moon, for example, an x-ray monitor was used to follow the possible variation in solar X-ray intensity and spectral shape while mapping the lunar surface with respect to its chemical composition due to the production of secondary X-rays.

The X-ray monitor of Solwind, designated NRL-608 or XMON, was a collaboration between the Naval Research Laboratory and Los Alamos National Laboratory. The monitor consisted of 2 collimated argon proportional counters. The instrument bandwidth of 3-10 keV was defined by the detector window absorption (the window was 0.254 mm beryllium) and the upper level discriminator. The active gas volume (P-10 mixture) was 2.54 cm deep, providing good efficiency up to 10 keV. Counts were recorded in 2 energy channels. Slat collimators defined a FOV of 3° x 30° (FWHM) for each detector; the long axes of the FOVs were perpendicular to each other. The long axes were inclined 45 degrees to the scan direction, allowing localization of transient events to about 1 degree. The centers of the FOVs coincided, and were pointed 40 degrees below the scan equator of the wheel in order to avoid scanning across the Sun. The spacecraft wheel rotated once every 6 seconds. This scan rate corresponds to 1 degree every 16 milliseconds; counts were telemetered in 64 or 32 millisecond bins to minimize smearing the collimator response.

The instrument parameters and data yield implied a 3 σ point source sensitivity of 30 UFU in one day's operation (1 UFU = 2.66 erg/cm-s-keV). Each detector was about 0.1 of the area of the Uhuru instrument. The instrument background at low geomagnetic latitudes was ~16 cts/s. Of this background, ~6 cts/s comes from the diffuse cosmic X-ray background, with the rest being instrumental. Assuming a conservative 10% data return, the net source duty cycle in scanning mode was 1.4 X 10, implying a source exposure of 120 seconds per day. For a background of 16 cts/s, the 3 σ error in determining the flux from a given sky bin was then 4.5 cts/s, or about 45 UFU, after 1 day. A limiting sensitivity of 30 UFU was obtained by combining both detectors. A comparable error existed in the flux determination for moderately bright galactic sources. Source confusion due to the 5° FOV projected along the scan direction complicated the observation of sources in the galactic bulge region (approximately 30 deg > l > -30 deg, |b| < 10 deg).

Scintillation detector

A scintillator is a material which exhibits the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, such as an X-ray photon, absorb its energy and scintillate, i.e. reemit the absorbed energy in the form of a small flash of light, typically in the visible range.

The scintillation X-ray detector (XC) aboard Vela 5A and its twin Vela 5B consisted of two 1 mm thick NaI(Tl) crystals mounted on photomultiplier tubes and covered by a 0.13 mm thick beryllium window. Electronic thresholds provided two energy channels, 3-12 keV and 6-12 keV. In front of each crystal was a slat collimator providing a full width at half maximum (FWHM) aperture of ~6.1 x 6.1 degrees. The effective detector area was ~26 cm. Sensitivity to celestial sources was severely limited by the high intrinsic detector background.

The X-ray telescope onboard OSO 4 consisted of a single thin NaI(Tl) scintillation crystal plus phototube assembly enclosed in a CsI(Tl) anti-coincidence shield. The energy resolution was 45 percent at 30 keV. The instrument operated from ~ 8 to 200 keV with 6 channel resolution.

OSO 5 carried a CsI crystal scintillator. The central crystal was 0.635 cm thick, had a sensitive area of 70 cm, and was viewed from behind by a pair of photomultiplier tubes. The shield crystal had a wall thickness of 4.4 cm and was viewed by 4 photomultipliers. The field of view was ~ 40 degrees. The energy range covered was 14-254 keV. There were 9 energy channels: the first covering 14-28 keV and the others equally spaced from 28-254 keV. In-flight calibration was done with an Am source.

The PHEBUS experiment recorded high energy transient events in the range 100 keV to 100 MeV. It consisted of two independent detectors and their associated electronics. Each detector consisted of a bismuth germinate (BGO) crystal 78 mm in diameter by 120 mm thick, surrounded by a plastic anti-coincidence jacket. The two detectors were arranged on the spacecraft so as to observe 4π steradians. The burst mode was triggered when the count rate in the 0.1 to 1.5 MeV energy range exceeded the background level by 8 σ (standard deviations) in either 0.25 or 1.0 seconds. There were 116 channels over the energy range.

The KONUS-B instrument consisted of seven detectors distributed around the spacecraft that responded to photons of 10 keV to 8 MeV energy. They consisted of NaI(Tl) scintillator crystals 200 mm in diameter by 50 mm thick behind a Be entrance window. The side surfaces were protected by a 5 mm thick lead layer. The burst detection threshold was 5 × 10 to 5 × 10 ergs/cm², depending on the burst spectrum and rise time. Spectra were taken in two 31-channel pulse height analyzers (PHAs), of which the first eight were measured with 1/16 s time resolution and the remaining with variable time resolutions depending on the count rate. The range of resolutions covered 0.25 to 8 s.

Kvant-1 carried the HEXE, or High Energy X-ray Experiment, which employed a phoswich of sodium iodide and cesium iodide. It covered the energy range 15-200 keV with a 1.6 deg x 1.6 deg FOV FWHM. Each of the 4 identical detectors had a geometric area of 200 cm. The maximum time resolution was 0.3-25 ms.

Modulation Collimator

Modulation is the process of varying one waveform in relation to another waveform. An X-ray collimator is a device that filters a stream of X-rays so that only those traveling parallel to a specified direction are allowed through.

Prof. Minoru Oda, President of Tokyo University of Information Sciences, invented the modulation collimator, first used to identify the counterpart of Sco X-1 in 1966, which led to the most accurate positions for X-ray sources available, prior to the launch of X-ray imaging telescopes.

SAS 3 carried modulation collimators (2-11 keV) and Slat and Tube collimators (1 up to 60keV).

On board the Granat International Astrophysical Observatory were four WATCH instruments that could localize bright sources in the 6 to 180 keV range to within 0.5° using a Rotation Modulation Collimator. Taken together, the instruments' three fields of view covered approximately 75% of the sky. The energy resolution was 30% FWHM at 60 keV. During quiet periods, count rates in two energy bands (6 to 15 and 15 to 180 keV) were accumulated for 4, 8, or 16 seconds, depending on onboard computer memory availability. During a burst or transient event, count rates were accumulated with a time resolution of 1 s per 36 s.

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Explorer 81, images solar flares from soft X-rays to gamma rays (~3 keV to ~20 MeV). Its imaging capability is based on a Fourier-transform technique using a set of 9 rotational modulation collimators.

X-ray spectrometer

OSO 8 had on board a Graphite Crystal X-ray Spectrometer, with energy range of 2-8 keV, FOV 3°.

The Granat ART-S X-ray spectrometer covered the energy range 3 to 100 keV, FOV 2° x 2°. The instrument consisted of four detectors based on spectroscopic MWPCs, making an effective area of 2,400 cm² at 10 keV and 800 cm² at 100 keV. The time resolution was 200 microseconds.

The X-ray spectrometer aboard ISEE-3 was designed to study both solar flares and cosmic gamma-ray bursts over the energy range 5-228 keV. The detector provided full-time coverage, 3π sr FOV for E > 130 keV, time resolution of 0.25 ms, and absolute timing to within 1 ms. It was intended to be a part of a long baseline interferometry network of widely separated spacecraft. The efforts were aimed primarily at determining the origin of the bursts through precise directional information established by such a network. The experiment consisted of 2 cylindrical X-ray detectors: a Xenon filled proportional counter covering 5-14 keV, and a NaI(Tl) scintillator covering 12-1250 keV. The proportional counter was 1.27 cm in diameter and was filled with a mixture of 97% Xenon and 3% carbon dioxide. The central part of the counter body was made of 0.51 mm thick beryllium and served as the X-ray entrance window. The scintillator consisted of a 1.0 cm thick cylindrical shell of NaI(Tl) crystal surrounded on all sides by 0.3 cm thick plastic scintillator. The central region, 4.1 cm in diameter, was filled by a quartz light pipe. The whole assembly was enclosed (except for one end) in a 0.1 cm thick beryllium container. The energy channel resolution and timing resolution could be selected by commands sent to the spacecraft. The proportional counter could have up to 9 channels with 0.5 s resolution; the NaI scintillator could have up to 16 channels and 0.00025 s resolution.

CCDs

Most existing X-ray telescopes use CCD detectors, similar to those in visible-light cameras. In visible-light, a single photon can produce a single electron of charge in a pixel, and an image is built up by accumulating many such charges from many photons during the exposure time. When an X-ray photon hits a CCD, it produces enough charge (hundreds to thousands of electrons, proportional to its energy) that the individual X-rays have their energies measured on read-out.

Microcalorimeters

Microcalorimeters can only detect x-rays one photon at a time (but can measure the energy of each). This works well for ground-based astronomical uses as few x-ray photons reach the earth, even from the strongest sources such as black holes See Microcalorimeters and X-ray microcalorimeter

Transition Edge Sensors

TES devices are the next step in microcalorimetery. In essence they are super-conducting metals kept as close as possible to their transition temperature. This is the temperature at which these metals become super-conductors and their resistance drops to zero. These transition temperatures are usually just a few degrees above absolute zero (usually less than 10 K).

Astronomical sources of X-rays

Several types of astrophysical objects emit X-rays, from galaxy clusters, through black holes in active galactic nuclei (AGN) to galactic objects such as supernova remnants, stars, and binary stars containing a white dwarf (cataclysmic variable stars and super-soft x-ray sources), neutron star or black hole (X-ray binaries). Some solar system bodies emit X-rays, the most notable being the Moon, although most of the X-ray brightness of the Moon arises from reflected solar X-rays. A combination of many unresolved X-ray sources is thought to produce the observed X-ray background. The X-ray continuum can arise from bremsstrahlung, either magnetic or ordinary Coulomb, black-body radiation, synchrotron radiation, inverse Compton scattering of lower-energy photons be relativistic electrons, knock-on collisions of fast protons with atomic electrons, and atomic recombination, with or without additional electron transitions.

Galaxy clusters

Clusters of galaxies are formed by the merger of smaller units of matter, such as galaxy groups or individual galaxies. The infalling material (which contains galaxies, gas and dark matter) gains kinetic energy as it falls into the cluster's gravitational potential well. The infalling gas collides with gas already in the cluster and is shock heated to between 10 and 10 K depending on the size of the cluster. This very hot gas emits X-rays by thermal bremsstrahlung emission, and line emission from metals (in astronomy, 'metals' often means all elements except hydrogen and helium). The galaxies and dark matter are collisionless and quickly become virialised, orbiting in the cluster potential well.

At a statistical significance of 8σ, it was found that the spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law.

Quasars

A quasi-stellar radio source (quasar) is a very energetic and distant galaxy with an active galactic nucleus (AGN). QSO 0836+7107 is a Quasi-Stellar Object that emits baffling amounts of radio energy. The radio signal is caused by electrons spiraling along the magnetic fields. These electrons can also interact with visible light emitted by the disk around the AGN or the black hole at its center, and that pumps them to emit X- and gamma-radiation.

On board the Compton Gamma Ray Observatory (CGRO) is the Burst and Transient Source Experiment (BATSE) which detects in the 20 keV to 8 MeV range. QSO 0836+7107 or 4C 71.07 was detected by BATSE as a source of soft gamma rays and hard X-rays. "What BATSE has discovered is that it can be a soft gamma-ray source," McCollough said. QSO 0836+7107 is the faintest and most distant object to be observed in soft gamma rays. It has already been observed in gamma rays by the Energetic Gamma Ray Experiment Telescope (EGRET) also aboard the Compton Gamma Ray Observatory.

Seyfert galaxies

Seyfert galaxies are a class of galaxies with nuclei that produce spectral line emission from highly ionized gas. They are a subclass of active galactic nuclei (AGN), and are thought to contain supermassive black holes.

Ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) are pointlike, nonnuclear X-ray sources with luminosities above the Eddington limit of 3 x 10 ergs s for a 20 M_☉ black hole. Many ULXs show strong variability and may be black hole binaries. To fall into the class of intermediate-mass black holes (IMBHs), their luminosities, thermal disk emissions, variation timescales, and surrounding emission-line nebulae must suggest this. However, when the emission is beamed or exceeds the Eddington limit, the ULX may be a stellar-mass black hole. The nearby spiral galaxy NGC 1313 has two compact ULXs, X-1 and X-2. For X-1 the X-ray luminosity increases to a maximum of 3 x 10 ergs s, exceeding the Eddington limit, and enters a steep power-law state at high luminosities more indicative of a stellar-mass black hole, whereas X-2 has the opposite behavior and appears to be in the hard X-ray state of an IMBH.

Black holes

Black holes give off radiation because matter falling into them loses gravitational energy which may result in the emission of radiation before the matter falls into the event horizon. The infalling matter has angular momentum, which means that the material cannot fall in directly, but spins around the black hole. This material often forms an accretion disk. Similar luminous accretion disks can also form around white dwarfs and neutron stars, but in these the infalling gas releases additional energy as it slams against the high-density surface with high speed. In case of a neutron star, the infall speed can be a sizeable fraction of the speed of light.

Supernova remnants (SNR)

A Type Ia supernova is an explosion of a white dwarf in orbit around either another white dwarf or a red giant star. The dense white dwarf can accumulate gas donated from the companion. When the dwarf reaches the critical mass of 1.4 solar masses, a thermonuclear explosion ensues. As each Type Ia shines with a known luminosity, Type Ia are called "standard candles" and are used by astronomers to measure distances in the universe.

SN 2005ke is the first Type Ia supernova detected in X-ray wavelengths, and it is much brighter in the ultraviolet than expected.

X-ray emission from Stars

In some neutron star or white dwarf systems, the magnetic field of the star is strong enough to prevent the formation of an accretion disc. The material in the disc gets very hot because of friction, and emits X-rays. The material in the disc slowly loses its angular momentum and falls into the compact star. In neutron stars and white dwarfs, additional X-rays are generated when the material hits their surfaces. X-ray emission from black holes is variable, varying in luminosity in very short timescales. The variation in luminosity can provide information about the size of the black hole.

Vela X-1

Vela X-1 is a pulsing, eclipsing high-mass X-ray binary (HMXB) system, associated with the Uhuru source 4U 0900-40 and the supergiant star HD 77581. The X-ray emission of the neutron star is caused by the capture and accretion of matter from the stellar wind of the supergiant companion. Vela X-1 is the prototypical detached HMXB.

Hercules X-1

An intermediate-mass X-ray binary (IMXB) is a binary star system where one of the components is a neutron star or a black hole. The other component is an intermediate mass star.

Her X-1 is composed of a neutron star accreting matter from a normal star (HZ Her) probably due to Roche lobe overflow. Her X-1 is the prototype for the massive X-ray binaries although it falls on the borderline, ~2 M_☉, between high- and low-mass X-ray binaries.

Scorpius X-1

The first extrasolar X-ray source was discovered on June 12, 1962. This source is called Scorpius X-1, the first X-ray source found in the constellation of Scorpius, located in the direction of the center of the Milky Way. Scorpius X-1 is some 9,000 ly from Earth and after the Sun is the strongest x-ray source in the sky at energies below 20 keV. Its X-ray output is 2.3 x 10 W, about 60,000 times the total luminosity of the Sun. Scorpius X-1 itself is a neutron star. This system is classified as a low-mass X-ray binary (LMXB); the neutron star is roughly 1.4 solar masses, while the donor star is only 0.42 solar masses.

Sun

In the late 1930s, the presence of a very hot, tenuous gas surrounding the Sun was inferred indirectly from optical coronal lines of highly ionized species. In the mid-1940s radio observations revealed a radio corona around the Sun. After detecting X-ray photons from the Sun in the course of a rocket flight, T. Burnight wrote, “The sun is assumed to be the source of this radiation although radiation of wave-length shorter than 4 angstroms would not be expected from theoretical estimates of black body radiation from the solar corona.” And, of course, people have seen the solar corona in scattered visible light during solar eclipses.

While neutron stars and black holes are the quintessential point sources of X-rays, all main sequence stars are likely to have hot enough coronae to emit X-rays. A- or F-type stars have at most thin convection zones and thus produce little coronal activity.

Similar solar cycle-related variations are observed in the flux of solar X-ray and UV or EUV radiation. Rotation is one of the primary determinants of the magnetic dynamo, but this point could not be demonstrated by observing the Sun: the Sun’s magnetic activity is in fact strongly modulated (due to the 11-year magnetic spot cycle), but this effect is not directly dependent on the rotation period.

Solar flares usually follow the solar cycle. CORONAS-F was launched on Jul 31 2001 to coincide with the 23rd solar cycle maximum. The solar flare of Oct 29 2003 showed a significant degree of polarization (> 70% in channels E2 = 40-60 keV and E3 = 60-100 keV, but only about 50% in E1 = 20-40 keV) in hard X-rays.

Coronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle, it is for this reason coronal loops are often found with sunspots at their footpoints. Coronal loops populate both active and quiet regions of the solar surface. The Yohkoh Soft X-ray Telescope (SXT) observed X-rays in the 0.25-4.0 keV range, resolving solar features to 2.5 arc seconds with a temporal resolution of 0.5-2 seconds. SXT was sensitive to plasma in the 2-4 MK temperature range, making it an ideal observational platform to compare with data collected from TRACE coronal loops radiating in the EUV wavelengths.

Variations of solar-flare emission in soft X-rays (10-130 nm) and EUV (26-34 nm) recorded onboard CORONAS-F demonstrate for most flares observed by CORONAS-F in 2001-2003 UV radiation preceded X-ray emission by 1-10 min.

White dwarfs

When the core of a medium mass star contracts, it causes a release of energy that makes the envelope of the star expand. This continues until the star finally blows its outer layers off. The core of the star remains intact and becomes a white dwarf. The white dwarf is surrounded by an expanding shell of gas in an object known as a planetary nebula. Planetary nebulae seem to mark the transition of a medium mass star from red giant to white dwarf. X-ray images reveal clouds of multimillion degree gas that have been compressed and heated by the fast stellar wind. Eventually the central star collapses to form a white dwarf. For a billion or so years after a star collapses to form a white dwarf, it is "white" hot with surface temperatures of ~20,000 K.

X-ray emission has been detected from PG 1658+441, a hot, isolated, magnetic white dwarf, first detected in an Einstein IPC observation and later identified in an Exosat channel multiplier array observation. "The broad-band spectrum of this DA white dwarf can be explained as emission from a homogeneous, high-gravity, pure hydrogen atmosphere with a temperature near 28,000 K." These observations of PG 1658+441 support a correlation between temperature and helium abundance in white dwarf atmospheres.

A super soft X-ray source (SSXS) radiates soft X-rays in the range of 0.09 to 2.5 keV. Super soft X-rays are believed to be produced by steady nuclear fusion on a white dwarf's surface of material pulled from a binary companion. This requires a flow of material sufficiently high to sustain the fusion.

Real mass transfer variations may be occurring in V Sge similar to SSXS RX J0513.9-6951 as revealed by analysis of the activity of the SSXS V Sge where episodes of long low states occur in a cycle of ~400 days.

RX J0648.0-4418 is an X-ray pulsator in the Crab nebula. HD 49798 is a subdwarf star that forms a binary system with RX J0648.0-4418. The subdwarf star is a bright object in the optical and UV bands. The orbital period of the system is accurately known. Recent XMM-Newton observations timed to coincide with the expected eclipse of the X-ray source allowed an accurate determination of the mass of the X-ray source (at least 1.2 solar masses), establishing the X-ray source as a rare, ultra-massive white dwarf.

Brown dwarfs

According to theory, an object that has a mass of less than about 8 percent of the mass of the Sun cannot sustain significant nuclear fusion in its core. This marks the dividing line between red dwarf stars and brown dwarfs. The dividing line between planets and brown dwarfs occurs with objects that have masses below about 1 percent of the mass of the Sun, or 10 times the mass of Jupiter. These objects cannot fuse deuterium.

LP 944-20

With no strong central nuclear energy source, the interior of a brown dwarf is in a rapid boiling, or convective state. When combined with the rapid rotation that most brown dwarfs exhibit, convection sets up conditions for the development of a strong, tangled magnetic field near the surface. The flare observed by Chandra from LP 944-20 could have its origin in the turbulent magnetized hot material beneath the brown dwarf's surface. A sub-surface flare could conduct heat to the atmosphere, allowing electric currents to flow and produce an X-ray flare, like a stroke of lightning. The absence of X-rays from LP 944-20 during the non flaring period is also a significant result. It sets the lowest observational limit on steady X-ray power produced by a brown dwarf star, and shows that coronas cease to exist as the surface temperature of a brown dwarf cools below about 2500° C and becomes electrically neutral.

TWA 5B

Using NASA's Chandra X-ray Observatory, scientists have detected X-rays from a low mass brown dwarf in a multiple star system. This is the first time that a brown dwarf this close to its parent star(s) (Sun-like stars TWA 5A) has been resolved in X-rays. "Our Chandra data show that the X-rays originate from the brown dwarf's coronal plasma which is some 3 million degrees Celsius," said Yohko Tsuboi of Chuo University in Tokyo. "This brown dwarf is as bright as the Sun today in X-ray light, while it is fifty times less massive than the Sun," said Tsuboi. "This observation, thus, raises the possibility that even massive planets might emit X-rays by themselves during their youth!"

X-ray reflection

Electric voltages of about 10 million volts, and currents of 10 million amps - a hundred times greater than the most powerful lightning bolts - are required to explain the auroras at Jupiter's poles, which are a thousand times more powerful than those on Earth.

On Earth, auroras are triggered by solar storms of energetic particles, which disturb Earth's magnetic field. As shown by the swept-back appearance in the illustration, gusts of particles from the Sun also distort Jupiter's magnetic field, and on occasion produce auroras.

Saturn's X-ray spectrum is similar to that of X-rays from the Sun indicating that Saturn's X-radiation is due to the reflection of solar X-rays by Saturn's atmosphere. The optical image is much brighter, and shows the beautiful ring structures, which were not detected in X-rays.

X-ray fluorescence

Some of the x-rays detected as originating from solar system bodies other than the Sun are produced by fluorescence. Scattered solar X-rays provide an additional component.

In the Röntgensatellit (ROSAT) image of the Moon, pixel brightness corresponds to x-ray intensity. The bright lunar hemisphere shines in x-rays because it re-emits x-rays originating from the sun. The background sky has an x-ray glow in part due to the myriad of distant, powerful active galaxies, unresolved in the ROSAT picture. The dark side of the Moon's disk shadows this X-ray background radiation coming from the deep space. A few x-rays only seem to come from the shadowed lunar hemisphere. Instead, they originate in Earth's geocorona or extended atmosphere which surrounds the orbiting x-ray observatory. The measured lunar x-ray luminosity of ~ 1.2 x 10 erg/s makes the Moon one of the weakest known non-terrestrial X-ray source.

Comet detection

NASA's Swift Gamma-ray Explorer satellite was monitoring Comet Lulin as it closed to 63 Gm of Earth. For the first time, astronomers can see simultaneous UV and X-ray images of a comet. "The solar wind -- a fast-moving stream of particles from the sun -- interacts with the comet's broader cloud of atoms. This causes the solar wind to light up with X rays, and that's what Swift's XRT sees," said Stefan Immler, of the Goddard Space Flight Center. This interaction, called charge exchange, results in X-rays from most comets when they pass within about three times Earth's distance from the sun. Because Lulin is so active, its atomic cloud is especially dense. As a result, the X-ray-emitting region extends far sunward of the comet.

Celestial X-ray sources

The celestial sphere has been divided into 88 constellations. The IAU constellations are areas of the sky. Each of these contains remarkable X-ray sources. Some of them are galaxies or black holes at the centers of galaxies. Some are pulsars. As with the astronomical X-ray sources, striving to understand the generation of X-rays by the apparent source helps to understand the Sun, the universe as a whole, and how these affect us on Earth.

Andromeda

Multiple X-ray sources have been detected in the Andromeda Galaxy, using observations from the ESA's XMM-Newton orbiting observatory.

Cassiopeia

Regarding Cas A SNR, it is believed that first light from the stellar explosion reached Earth approximately 300 years ago but there are no historical records of any sightings of the progenitor supernova, probably due to interstellar dust absorbing optical wavelength radiation before it reached Earth (although it is possible that it was recorded as a sixth magnitude star 3 Cassiopeiae by John Flamsteed on August 16, 1680). Possible explanations lean toward the idea that the source star was unusually massive and had previously ejected much of its outer layers. These outer layers would have cloaked the star and reabsorbed much of the light released as the inner star collapsed.

CTA 1 is another SNR X-ray source in Cassiopeia. A pulsar in the CTA 1 supernova remnant (4U 0000+72) initially emitted radiation in the X-ray bands (1970-1977). Strangely, when it was observed at a later time (2008) X-ray radiation was not detected. Instead, the Fermi Gamma-ray Space Telescope detected the pulsar was emitting gamma ray radiation, the first of its kind.

Cetus

Abell 400 is a galaxy cluster, containing a galaxy (NGC 1128) with two supermassive black holes 3C 75 spiralling towards merger.

Sagittarius

The Galactic Center is at 1745-2900 which corresponds to Sagittarius A*, very near to radio source Sagittarius A (W24). In probably the first catalogue of galactic X-ray sources, two Sgr X-1s are suggested: (1) at 1744-2312 and (2) at 1755-2912, noting that (2) is an uncertain identification. Source (1) seems to correspond to S11.

Serpens

As of Aug 27 2007 discoveries concerning asymmetric iron line broadening and their implications for relativity have been a topic of much excitement. With respect to the asymmetric iron line broadening, Edward Cackett of the University of Michigan commented, "We're seeing the gas whipping around just outside the neutron star's surface,". "And since the inner part of the disk obviously can't orbit any closer than the neutron star's surface, these measurements give us a maximum size of the neutron star's diameter. The neutron stars can be no larger than 18 to 20.5 miles across, results that agree with other types of measurements."

"We've seen these asymmetric lines from many black holes, but this is the first confirmation that neutron stars can produce them as well. It shows that the way neutron stars accrete matter is not very different from that of black holes, and it gives us a new tool to probe Einstein's theory," says Tod Strohmayer of NASA's Goddard Space Flight Center.

"This is fundamental physics," says Sudip Bhattacharyya also of NASA's Goddard Space Flight Center in Greenbelt, MD and the University of Maryland. "There could be exotic kinds of particles or states of matter, such as quark matter, in the centers of neutron stars, but it's impossible to create them in the lab. The only way to find out is to understand neutron stars."

Using XMM-Newton, Bhattacharyya and Strohmayer observed Serpens X-1, which contains a neutron star and a stellar companion. Cackett and Jon Miller of the University of Michigan, along with Bhattacharyya and Strohmayer, used Suzaku's superb spectral capabilities to survey Serpens X-1. The Suzaku data confirmed the XMM-Newton result regarding the iron line in Serpens X-1.

Ursa Major

M82 X-1 is in the constellation Ursa Major at 09 55 50.01 +69° 40′ 46.0″. It was detected in January 2006 by the Rossi X-ray Timing Explorer.

Forthcoming X-ray observatory satellites

Agreement signed by the Russian Federal Space Agency (Roscosmos) and the German Aerospace Center (DLR)

Among the contracts negotiated in August at the MAKS International Aviation and Space Salon there was an agreement signed by the Russian Federal Space Agency (Roscosmos) and the German Aerospace Center (DLR). The contract details the creation of the Orbital Astrophysics Observatory Spectrum-X-Gamma (SXG) planned to be launched in 2012.

According to Mikhail Pavlinsky, deputy head of the Space Research Institute (SPI), the total project cost nears €50 million. Under the agreement, Germany will provide the main of the two X-ray telescopes, while Russia will install it on its platform, prepare the spacecraft, and take care of all related issues. Russia will also install an additional telescope on this platform.

The project is unique not only because of its participants and large scale, but also because of its tasks and possible discoveries. The project will be looking for galaxies, not stars. When it comes to discovering further galaxies, even the telescopes brought beyond the Earth’s atmosphere are sometimes blind when compared to X-ray telescopes. Space is permeated by dust and at times it is hard for X-rays to get through it. The new observatory will help scientists perform an all-sky scan survey.9

Constellation-X

Constellation-X will provide high resolution X-ray spectroscopy to probe matter as it falls into a black hole, as well as probe the nature of dark matter and dark energy by observing the formation of clusters of galaxies. The International X-ray Observatory (IXO) – a joint effort of NASA, ESA, and JAXA supersedes the Constellation-X mission.

International X-ray Observatory

International X-ray Observatory (IXO) is the result of the merging of NASA´s Constellation-X and ESA/JAXA´s XEUS mission concepts. It will feature a single large X-ray mirror with a 3 square meter collecting area and 5 arcsec angular resolution, and a suite of instrumentation, including a wide field imaging detector, a hard X-ray imaging detector, a high-spectral-resolution imaging spectrometer (calorimeter), a grating spectrometer, a high timing resolution spectrometer, and a polarimeter. Launch is planned for 2021.

ESA Solar Orbiter

The Solar Orbiter will approach to 48 solar radii to view the solar atmosphere with high spatial resolution in visible, XUV, and X-rays. The nominally 6 yr mission will be form an elliptical orbit around the Sun with perihelion as low as 0.23 AU and with increasing inclination up to more than 30° with respect to the solar equator. The Orbiter will deliver images and data from the polar regions and the side of the Sun not visible from Earth. The launch date is to be determined (TBD).

Explorational X-ray astronomy

Usually observational astronomy is considered to occur on Earth's surface (or beneath it in neutrino astronomy). This idea of limiting observation to Earth includes orbiting the Earth. As soon as the observer leaves the cozy confines of Earth, the observer becomes an explorer. Except for Explorer 1 and Explorer 3 and the earlier satellites in the series, usually if it's going to be an explorer it leaves the Earth or Earth orbit. To qualify as an X-ray astronomer/explorer or "astronobot"/explorer, carry an XRT or X-ray detector on board and leave Earth orbit.

Luna 17 was launched from an Earth parking orbit towards the Moon and entered lunar orbit on Nov 15 1970. The spacecraft had dual ramps by which Lunokhod 1 descended to the lunar surface. Lunokhod 1 carried onboard an X-ray telescope, but may not have made any observations with it.

Lunokhod 2 was carried to the Moon by Luna 21. Like Lunokhod 1, it carried onboard a X-ray telescope. It had a mission to observe solar X-rays but may not have made any observations.

ISEE-3 was launched on Aug 12 1978. It was inserted into a "halo" orbit about the libration point some 240 Earth radii upstream between the Earth and Sun. ISEE-3 was renamed ICE (International Cometary Explorer) when, after completing its original mission in 1982, it was gravitationally maneuvered to intercept the comet P/Giacobini-Zinner. On September 11, 1985, the veteran NASA spacecraft flew through the tail of the comet. The X-ray spectrometer aboard ISEE-3 was designed to study both solar flares and cosmic gamma-ray bursts over the energy range 5-228 keV.

Several of the Venera program flyby probes (Venera 11, 12 and 14) carried an X-ray/γ-ray detector that observed extrasolar X-rays, while enroute to Venus or in heliocentric orbit.

Venera 11 and Venera 12 flyby buses each carried a Sneg-2MZ gamma- and x-ray burst detector. Venera 11 arrived at Venus after two course corrections on Sep 16 and Dec 17 1978 and separated on Dec 23 1978 from the lander. The flyby probe entered heliocentric orbit after flying past the planet at a range of 35,000 km. Venera 12 was the identical sister craft to Venera 11. The spacecraft performed two midcourse corrections on Sep 21 and Dec 14 1978, and as with its twin, two days prior to the planetary encounter, the flyby probe released its lander. It also entered heliocentric orbit. During their mission lifetimes Venera 11 & 12 did observe X-rays from extrasolar sources.

The Venera 14 flyby probe carried a Signe-2MS3 gamma-ray burst detector which was used to provide data on solar X-ray flares. The flyby probe separated from the lander on Mar 3 1982, passed Venus at a range of 36,000 kilometers and entered heliocentric orbit. During its mission lifetime Venera 14 did observe X-rays from an extrasolar source.

The Phobos program consisted of Phobos 1 and 2 which carried X-ray telescopes with RF-15 X-ray spectrometer to conduct studies of the interplanetary environment, perform observations of the Sun, and characterize the plasma environment in the Martian vicinity.

Ulysses was launched Oct 6 1990, and reached Jupiter for its "gravitational slingshot" in Feb 1992. It passed the south solar pole in Jun 1994 and crossed the ecliptic equator in Feb 1995. The solar X-ray and cosmic gamma-ray burst experiment (GRB) had 3 main objectives: study and monitor solar flares, detect and localize cosmic gamma-ray bursts, and in-situ detection of Jovian aurorae. Ulysses was the first satellite carrying a gamma burst detector which went outside the orbit of Mars. The hard X-ray detectors operated in the range 15-150 keV. The detectors consisted of 2 3-mm thick x 51-mm diameter CsI(Tl) crystals mounted via plastic light tubes to photomultipliers. The hard detector changed its operating mode depending on (1) measured count rate, (2) ground command, or (3) change in spacecraft telemetry mode. The trigger level was generally set for 8-sigma above background and the sensitivity is ~1 erg/cm. When a burst trigger is recorded, the instrument switches to record high resolution data, recording it to a 32-kbit memory for a slow telemetry read out. Burst data consist of either 16 s of 8-ms resolution count rates or 64 s of 32-ms count rates from the sum of the 2 detectors. There were also 16 channel energy spectra from the sum of the 2 detectors (taken either in 1,2,4,16, or 32 second integrations). During 'wait' mode, the data were taken either in 0.25 or 0.5 s integrations and 4 energy channels (with shortest integration time being 8 s). Again, the outputs of the 2 detectors were summed.

The Ulysses soft X-ray detectors consisted of 2 500-micron thick x 0.5 cm area Si surface barrier detectors. A 100 mg/cm beryllium foil front window rejected the low energy X-rays and defined a conical FOV of 75° (half-angle). These detectors were passively cooled and operate in the temperature range -35 to -55 degrees Celsius. This detector had 6 energy channels, covering the range 5-20 keV.

WIND was launched on Nov 1 1994. At first, the satellite had a lunar swingby orbit around the Earth. With the assistance of the Moon's gravitational field Wind's apogee was kept over the day hemisphere of the Earth and magnetospheric observations were made. Later in the mission, the Wind spacecraft was inserted into a special "halo" orbit in the solar wind upstream from the Earth, about the sunward Sun-Earth equilibrium point (L1). The satellite has a spin period of ~ 20 seconds, with the spin axis normal to the ecliptic. WIND carries the Transient Gamma-Ray Spectrometer (TGRS) which covers the energy range 15 keV - 10 MeV, with an energy resolution of 2.0 keV @ 1.0 MeV (E/delta E = 500). The onboard KONUS experiment provides event time profiles in three energy ranges, from 10 to 770 keV, with 64-millisecond time resolution.

Theoretical X-ray astronomy

Like theoretical astrophysics, theoretical X-ray astronomy uses a wide variety of tools which include analytical models (for example, polytropes to approximate the behaviors of a star) and computational numerical simulations. Once potential observational consequences are available they can be compared with experimental observations. Observers can look for data that refutes a model or helps in choosing between several alternate or conflicting models.

Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.

Most of the topics in astrophysics, astrochemistry, astrometry, and other fields that are branches of astronomy studied by theoreticians involve X-rays and X-ray sources. Many of the beginnings for a theory can be found in an Earth-based laboratory where an X-ray source is built and studied.

Dynamos

If some of the stellar magnetic fields are really induced by dynamos, then field strength might be associated with rotation rate.

Astronomical models

From the observed X-ray spectrum, combined with spectral emission results for other wavelength ranges, an astronomical model addressing the likely source of X-ray emission can be constructed. For example, with Scorpius X-1 the X-ray spectrum steeply drops off as X-ray energy increases up to 20 keV, which is likely for a thermal-plasma mechanism. In addition, there is no radio emission, and the visible continuum is roughly what would be expected from a hot plasma fitting the observed X-ray flux. The plasma could be a corona to a central object or a transient plasma, where the energy source is unknown, but could be related to the idea of a close binary.

In the Crab Nebula X-ray spectrum there are three features that differ greatly from Scorpius X-1: its spectrum is much harder, its source diameter is in ly, not AU, and its radio and optical synchrotron emission are strong. Its overall X-ray luminosity rivals the optical emission and could be that of a nonthermal plasma. But the Crab Nebula appears as an X-ray source that is a central freely expanding ball of dilute plasma, where the energy content is 100 times the total energy content of the large visible and radio portion, obtained from the unknown source.

The "Dividing Line" as giant stars evolve to become red giants also coincides with the Wind and Coronal Dividing Lines. To explain the drop in X-ray emission across these dividing lines, a number of models have been proposed:

1. low transition region densities, leading to low emission in coronae,
2. high-density wind extinction of coronal emission,
3. only cool coronal loops become stable,
4. change in magnetic field structure to an open topology, leading to a decrease of magnetically confined plasma,
5. change in magnetic dynamo character, leading to the disappearance of stellar fields leaving only small-scale, turbulence-generated fields among red giants.

Analytical X-ray astronomy

High-mass X-ray binaries (HMXBs) are composed of an OB supergiant companion star and a compact object, usually a neutron star (NS) or black hole (BH). Supergiant X-ray binaries (SGXBs) are HMXBs in which the compact object orbits the massive companion within a few days (3-15 d) in circular (or slightly eccentric) orbits. SGXBs show typical hard X-ray spectra of accreting pulsars and most show a strong absorption as obscured HMXBs. X-ray luminosity increases up to 10 erg s.

The mechanism triggering the different temporal behavior observed between the classical SGXBs and the recently discovered SFXTs is still debated.

Aim: use the discovery of long orbits (>15 d) to help discriminate between emission models and perhaps bring constraints on the models.

Method: analyze archival data on various SGXBs such as has been obtained by INTEGRAL for candidates exhibiting long orbits. Build short- and long-term light curves. Perform a timing analysis in order to study the temporal behavior of each candidate on different time scales.

Compare various astronomical models:

• direct spherical accretion
• Roche-Lobe overflow via an accretion disk on the compact object.

Draw some conclusions: for example, the SGXB SAX J1818.6-1703 was discovered by BeppoSAX in 1998, identified as a SGXB of spectral type between O9I−B1I, which also displayed short and bright flares and an unusually very low quiescent level leading to its classification as a SFXT. The analysis indicated an unusually long orbital period: 30.0 ± 0.2 d and an elapsed accretion phase of ~6 d implying an elliptical orbit and possible supergiant spectral type between B0.5-1I with eccentricities e ~ 0.3-0.4. The hugh variations in the X-ray flux can be explained through accretion of macro-clumps formed within the stellar wind.

Choose which model seems to work best: for SAX J1818.6-1703 the analysis best fits the model that predicts SFXTs behave as SGXBs with different orbital parameters; hence, different temporal behavior.

Stellar X-ray astronomy

Stellar X-ray astronomy started on Apr 5 1974 with the detection of X-rays from Capella. A rocket flight on that date briefly calibrated its attitude control system when a star sensor pointed the payload axis at Capella (α Aur). During this period, X-rays in the range 0.2-1.6 keV were detected by an X-ray reflector system co-aligned with the star sensor. The X-ray luminosity of ~10 erg s is four orders of magnitude above the Sun's X-ray luminosity.

Stellar coronae

Experiments with instruments aboard Skylab and Copernicus have been used to search for soft X-ray emission in the energy range ~0.14-0.284 keV from stellar coronae. The experiments aboard ANS succeeded in finding X-ray signals from Capella and Sirius (α CMa). X-ray emission from an enhanced solar-like corona was proposed for the first time. The high temperature of Capella's corona as obtained from the first coronal X-ray spectrum of Capella using HEAO 1 required magnetic confinement unless it was a free-flowing coronal wind.

Also, in 1967-68 the first stellar X-ray flares were observed on YZ CMi, AD Leo, EV Lac, and UV Cet in soft X-rays. The thermal nature of the emission for the first explicit X-ray spectra of extrasolar flares was confirmed by the detections of the 6.7 keV Fe Kα line.

Later in 1977 Proxima Centauri was discovered to be emitting high-energy radiation in the XUV. In 1978, α Cen was identified as a low-activity coronal source. With the operation of the Einstein observatory, X-ray emission was recognized as a characteristic feature common to a wide range of stars covering essentially the whole Hertzsprung-Russell diagram. The ROSAT All-Sky survey identified tens of thousands of coronal sources. The Einstein initial survey led to significant insights:

• X-ray sources abound among all types of stars, across the Hertzsprung-Russell diagram and across most stages of evolution,
• the X-ray luminosities and their distribution along the main sequence were not in agreement with the long-favored acoustic heating theories, but were now interpreted as the effect of magnetic coronal heating, and
• stars that are otherwise similar reveal large differences in their X-ray output if their rotation period is different.

To fit the medium-resolution spectrum of UX Ari, subsolar abundances were required.

X-ray activity in solar-like main sequence stars is strongly correlated with the period of stellar rotation. The faster the rotation, the higher the X-ray luminosity. Further, the higher the X-ray activity, the hotter the coronae.

Star formation regions as a whole, and individual stars such as T Tauri stars have been detected as strong and unexpectedly variable X-ray sources, including the presence of strong flares.

Coronae are ubiquitous among the stars in the cool half of the Hertzsprung-Russell diagram. Stellar X-ray astronomy is contributing toward a deeper understanding of

• magnetic fields in magnetohydrodynamic dynamos,
• the release of energy in tenuous astrophysical plasmas through various plasma-physical processes, and
• the interactions of high-energy radiation with the stellar environment.

Current wisdom has it that the massive coronal main sequence stars are late-A or early F stars, a conjecture that is supported both by observation and by theory.

Unstable winds

Given the lack of a significant outer convection zone, theory predicts the absence of a magnetic dynamo in earlier A stars. In early stars of spectral type O and B, shocks developing in unstable winds are the likely source of X-rays.

Coolest M dwarfs

Beyond spectral type M5, the classical αω dynamo can no longer operate as the internal structure of dwarf stars changes significantly: they become fully convective. As a distributed (or α) dynamo may become relevant, both the magnetic flux on the surface and the topology of the magnetic fields in the corona should systematically change across this transition, perhaps resulting in some discontinuities in the X-ray characteristics around spectral class dM5. But, observations do not seem to support this picture: long-time lowest-mass X-ray detection, VB 8 (M7e V), has shown steady emission at levels of X-ray luminosity (L_X) ~10 erg s and flares up to an order of magnitude higher. Comparison with other late M dwarfs shows a rather continuous trend.

The boundary toward the substellar regime (around masses of 0.07M_⊙) suggests a change in the magnetic behavior, for the following reason: the photospheres of such stars are dominated by molecular hydrogen, with a very low ionization degree of approximately 10. Electric currents flow parallel to the coronal magnetic field lines in the predominant non-flaring force-free configuration, but since currents cannot flow into the almost neutral photosphere, any equilibrium coronal configuration is not capable of liberating energy for heating, producing a precipitous drop of L_X.

Strong X-ray emission from Herbig Ae/Be stars

Herbig Ae/Be stars are pre-main sequence stars. As to their X-ray emission properties, some are

• reminescent of hot stars,
• others point to coronal activity as in cool stars, in particular the presence of flares and very high temperatures.

The nature of these strong emissions has remained controversial with models including

• unstable stellar winds,
• colliding winds,
• magnetic coronae,
• disk coronae,
• wind-fed magnetospheres,
• accretion shocks,
• the operation of a shear dynamo,
• the presence of unknown late-type companions.

K giants

The FK Com stars are giants of spectral type K with an unusually rapid rotation and signs of extreme activity. Their X-ray coronae are among the most luminous (L_X ≥ 10 erg s) and the hottest known with dominant temperatures up to 40 MK. But the current popular hypothesis involves a merger of a close binary system in which the orbital angular momentum of the companion is transferred to the primary.

Pollux is the brightest star in the constellation Gemini, despite its Beta designation, and the 17th brightest in the sky. Pollux is a giant orange K star that makes an interesting color contrast with its white "twin," Castor. Evidence has been found for a hot, outer, magnetically supported corona around Pollux, and the star is known to be an X-ray emitter.

Amateur X-ray astronomy

Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with equipment that they build themselves. The United States Air Force Academy (USAFA) is the home of the US's only undergraduate satellite program, and has and continues to develop the FalconLaunch sounding rockets. In addition to any direct amateur efforts to put X-ray astronomy payloads into space, there are opportunities that allow student-developed experimental payloads to be put on board commercial sounding rockets as a free-of-charge ride.

Self-built equipment

There are major limitations to amateurs observing and reporting experiments in X-ray astronomy: the cost of building an amateur rocket or balloon to place a detector high enough and the cost of appropriate parts to build a suitable X-ray detector.

Amateur rocketry

The Reaction Research Society on Nov 23 1996 launched a solid fueled rocket, designed by longtime member George Garboden, to an altitude of 50 miles (80 km) from Black Rock Desert in Nevada.

On May 17, 2004 Civilian Space eXploration Team (CSXT) successfully launched the first amateur high-power rocket into space, achieving an altitude of 72 miles (115 km).

Amateur ballooning

The Amateur Radio High Altitude Ballooning movement is a group of amateurs launching latex weather balloons to altitudes of 25 to 35 km. The usual flight time is around 2-3 hours, but experiments with zero-pressure balloons, superpressure balloons, and valved latex balloons have extended flight times to more than 24 hours.

Currently, payloads consist of tracking equipment, sensors, data loggers, cameras, amateur television (ATV) transmitters or other scientific experiments.

As yet no one has placed an X-ray detector onboard with appropriate orienting equipment.

Amateur detectors

CCDs are available for detectors and spectrographs. For normal incidence or glancing angle incidence, the main cost limitation is the telescope X-ray optics.

History of X-ray astronomy

In 1927, E.O. Hulburt of the US Naval Research Laboratory and associates Gregory Breit and Merle Tuve of the Carnegie Institution of Washington explored the possibility of equipping Robert H. Goddard's rockets to explore the upper atmosphere. "Two years later, he proposed an experimental program in which a rocket might be instrumented to explore the upper atmosphere, including detection of ultraviolet radiation and X rays at high altitudes."

In the late 1930s, the presence of a very hot, tenuous gas surrounding the Sun was inferred indirectly from optical coronal lines of highly ionized species. The Sun has been known to be surrounded by a hot tenuous corona. In the mid-1940s radio observations revealed a radio corona around the Sun.

The beginning of the search for X-ray sources from above the Earth's atmosphere was on Aug 5 1948 12:07 GMT. A US Army (formerly German) V-2 rocket as part of Project Hermes was launched from White Sands Proving Grounds. The first solar X-rays were recorded by T. Burnight. After detecting X-ray photons from the Sun in the course of the rocket flight, Burnight wrote, “The sun is assumed to be the source of this radiation although radiation of wave-length shorter than 4 angstroms would not be expected from theoretical estimates of black body radiation from the solar corona.”

Through the 1960s, 70s, 80s, and 90s, the sensitivity of detectors increased greatly during the 60 years of X-ray astronomy. In addition, the ability to focus X-rays has developed enormously -- allowing the production of high-quality images of many fascinating celestial objects.

Detecting X-ray sources

As of 2000, there were about 220,000 known X-ray sources, with the vast majority discovered by the ROSAT X-ray satellite observatory.

The USA V-2 period

The beginning of the search for X-ray sources from above the Earth's atmosphere was on Aug 5 1948 12:07 GMT. A US Army V-2 as part of Project Hermes was launched from White Sands Proving Grounds Launch Complex (LC) 33. In addition to carrying experiments of the US Naval Research Laboratory for cosmic and solar radiation, temperature, pressure, ionosphere, and photography, there was on board a solar X-ray test detector, which functioned properly. The missile reached an apogee of 166 km.

As part of a collaboration between the US Naval Research Laboratory (NRL) and the Signal Corps Engineering Laboratory (SCEL) of the University of Michigan, another V-2 (V-2 42 configuration) was launched from White Sands LC33 on Dec 9 1948 at 16:08 GMT (09:08 local time). The missile reached an apogee of 108.7 km and carried aeronomy (winds, pressure, temperature), solar X-ray and radiation, and biology experiments.

On Jan 28 1949 17:20 GMT an NRL X-ray detector (Blossom) was placed in the nose cone of a V-2 rocket and launched at White Sands Missile Range in New Mexico. X-rays from the Sun were detected. Apogee: 60 km.

A second collaborative effort (NRL/SCEL) using a V-2 UM-3 configuration launched on Apr 11 1949 at 22:05 GMT. Experiments included solar X-ray detection, apogee: 87.4 km.

NRL Ionosphere 1 solar X-ray, ionosphere, meteorite mission launched a V-2 on Sep 29 1949 from White Sands at 16:58 GMT and reached 151.1 km.

Using V-2 53 configuration a solar X-ray experiment was launched on Feb 17 1950 from White Sands LC 33 at 18:01 GMT reaching an apogee of 148 km.

The last V-2 launch number TF2/TF3 came on Aug 22 1952 07:33 GMT from White Sands reaching an apogee of 78.2 km and carried experiments

• solar X-ray for NRL,
• cosmic radiation for the National Institute of Health (NIH), and
• sky brightness for the Air Research and Development Command.

Aerobee period

The first successful launch of an Aerobee occurred on May 5 1952 13:44 GMT from White Sands Proving Grounds launch complex LC35. It was an Aerobee RTV-N-10 configuration reaching an apogee of 127 km with NRL experiments for solar X-ray and ultraviolet detection.

On Apr 19 1960 an Office of Naval Research Aerobee Hi made a series of X-ray photographs of the Sun from an altitude of 208 km. The mainstay of the US IGY rocket stable was the Aerobee Hi, which was modified and improved to create the Aerobee 150.

An Aerobee 150 rocket launched on Jun 12 1962 detected the first X-rays from other celestial sources (Scorpius X-1).

USSR V-2 derivative launches

Starting on Jun 21 1959, from Kapustin Yar, with a modified V-2 designated the R-5V, the USSR launched a series of four vehicles to detect solar X-rays: a R-2A on Jul 21 1959 and two R-11A at 02:00 GMT and 14:00 GMT.

Skylark

The British Skylark was probably the most successful of the many sounding rocket programs. The first launched in 1957 from Woomera, Australia and its 441st and final launch took place from Esrange, Sweden on 2 May 2005. Launches were carried out from sites in Australia, Europe, and South America, with use by NASA, the European Space Research Organisation (ESRO), and German and Swedish space organizations. Skylark was used to obtain the first good-quality X-ray images of the solar corona.

The first X-ray surveys of the sky in the Southern Hemisphere were provided by Skylark launches. It was also used with high precision in Sep and Oct 1972 in an effort to locate the optical counterpart of X-ray source GX3+1 by lunar occultation.

Véronique

The French Véronique was successfully launched on Apr 14 1964 from Hammaguira, LC Blandine carrying experiments to measure UV and X-ray intensities and the FU110 to measure UV intensity from the atomic H (Lyman-α) line, and again on Nov 4 1964.

Early Satellites

The SOLar RADiation satellite program (SOLRAD) was conceived in the late 1950s to study the Sun's effects on Earth, particularly during periods of heightened solar activity. Solrad 1 was launched on Jun 22 1960 aboard a Thor Able from Cape Canaveral at 1:54 a.m. EDT. As the world's first orbiting astronomical observatory, SOLRAD I determined that radio fade-outs were caused by solar X-ray emissions.

The first in a series of 8 successfully launched Orbiting Solar Observatories (OSO 1, launched on Mar 7 1963) had as its primary mission to measure solar electromagnetic radiation in the UV, X-ray, and gamma-ray regions.

OGO 1, the first of the Orbiting Geophysical Observatories (OGOs), was successfully launched from Cape Kennedy on Sep 5 1964 and placed into an initial orbit of 281 x 149,385 km at 31 degrees inclination. A secondary objective was to detect gamma-ray bursts from the Sun in the energy range 80 keV - 1 MeV. The experiment consisted of 3 CsI crystals surrounded by a plastic anti-coincidence shield. Once every 18.5 seconds, integral intensity measurements were made in each of 16 energy channels which were equally spaced over the .08-1 MeV range. OGO 1 was completely terminated on Nov 1 1971. Although the satellite did not achieve its goals due to electrical interference and secular degradation, searching back through the data after the discovery of cosmic gamma-ray bursts by the Vela satellites revealed the detection of one or more such events in the OGO 1 data.

Solar X-ray bursts were observed by OSO 2 and an effort was made to map the entire celestial sphere for direction and intensity of X-radiation.

The first USA satellite which detected cosmic X-rays was the Third Orbiting Solar Observatory, or OSO-3, launched on Mar 8 1967. It was intended primarily to observe the Sun, which it did very well during its 2 year lifetime, but it also detected a flaring episode from the source Sco X-1 and measured the diffuse cosmic X-ray background.

The fourth successful Orbiting Solar Observatory, OSO 4, was launched on Oct 18 1967. The objectives of the OSO 4 satellite were to perform solar physics experiments above the atmosphere and to measure the direction and intensity over the entire celestial sphere in UV, X, and gamma radiation. The OSO 4 platform consisted of a sail section (which pointed 2 instruments continuously toward the Sun) and a wheel section which spun about an axis perpendicular to the pointing direction of the sail (which contained 7 experiments). The spacecraft performed normally until a second tape recorder failed in May 1968. OSO 4 was put into a "standby" mode in Nov 1969. It could be turned on only for recording special events in real-time. One such event occurred on Mar 7 1970 during a solar eclipse. The spacecraft became totally inoperable on Dec 7 1971.

OGO 5 was launched on Mar 4 1968. The satellite, primarily devoted to Earth observation, was in a highly elliptical initial orbit with a 272 km perigee and an 148,228 km apogee. The orbital inclination was 31.1 degrees. The satellite took 3796 minutes to complete one orbit. The Energetic Radiations from Solar Flares experiment was operational from Mar 1968 - Jun 1971. Primarily devoted to solar observations, it detected at least 11 cosmic X-ray bursts in time coincidence with gamma-ray bursts seen by other instruments. The detector was a 0.5 cm thick NaI(Tl) crystal with a 9.5 cm area. Data were accumulated into energy ranges of: 9.6-19.2, 19.2-32, 32-48, 48-64, 64-80, 80-104, 104-128, and > 128 keV. The data were sampled for 1.15 seconds once every 2.3 seconds.

Cosmos 215 was launched Apr 19 1968 and contained an X-ray experiment. Orbit characteristics: 261 x 426 km, at an inclination of 48.5 degrees. The orbital period was ~ 91 minutes. It was intended primarily to perform solar studies, but did detect some non-solar X-ray events. It reentered the atmosphere on Jun 30 1968.

OSO 5 was launched on Jan 22 1969, and lasted until Jul 1975. It was the 5th satellite put into orbit as part of the Orbiting Solar Observatory program. This program was intended to launch a series of nearly identical satellites to cover an entire 11-year solar cycle. The circular orbit had an altitude of 555 km and an inclination of 33 degrees. The spin rate of the satellite was 1.8 s. The data produced a spectrum of the diffuse background over the energy range 14-200 keV.

OSO 6 was launched on Aug 9 1969. Its orbital period was ~95 min. The spacecraft had a spin rate of 0.5 rps. On board was a hard X-ray detector (27-189 keV) with a 5.1 cm NaI(Tl) scintillator, collimated to 17° x 23° FWHM. The system had 4 energy channels (separated 27-49-75-118-189 keV). The detector spun with the spacecraft on a plane containing the Sun direction within ± 3.5°. Data were read with alternate 70 ms and 30 ms integrations for 5 intervals every 320 ms.

Like the previous Vela 5 satellites, the Vela 6 nuclear test detection satellites were part of a program run jointly by the Advanced Research Projects of the U. S. Department of Defense and the U. S. Atomic Energy Commission, managed by the U. S. Air Force. The twin spacecraft, Vela 6A and 6B, were launched on Apr 8 1970. Data from the Vela 6 satellites were used to look for correlations between gamma-ray bursts and X-ray events. At least 2 good candidates were found, GB720514 and GB740723. The X-ray detectors failed on Vela 6A on Mar 12 1972 and on Vela 6B on Jan 27 1972.

Cosmos 428 was launched by the USSR into Earth orbit on Jun 24 1971 and recovered Jul 6 1971. The orbit characteristics: apogee/perigee/inclination 208 km, 271 km, and 51.8 degrees, respectively. It was a military satellite on which X-ray astronomy experiments had been added. There was a scintillation spectrometer sensitive to X-rays >30 keV, with a 2 deg x 17 deg field of view. In addition, there was an X-ray telescope which operated in the range 2-30 keV. Cosmos 428 detected several X-ray sources which were correlated to already identified Uhuru point sources.

To continue the intensive X-ray investigation of the Sun and the cosmic X-ray background OSO 7 was launched on Sep 29 1971. OSO 7 made the first observation of solar gamma-ray line emission, due to electron/positron annihilation at 511 keV, from a solar flare in Apr 1972.

TD-1A was put in a nearly circular polar sun-synchronous orbit, with apogee 545 km, perogee 533 km, and inclination 97.6°. It was ESRO's first 3-axis stabilized satellite, with one axis pointing to the Sun to within ±5 degrees. The optical axis was maintained perpendicular to the solar pointing axis and to the orbital plane. It scanned the entire celestial sphere every 6 months, with a great circle being scanned every satellite revolution. After about 2 months of operation, both of the satellite's tape recorders failed. A network of ground stations was put together so that real-time telemetry from the satellite was recorded for about 60% of the time. After 6 months in orbit, the satellite entered a period of regular eclipses as the satellite passed behind the Earth -- cutting off sunlight to the solar panels. The satellite was put into hibernation for 4 months, until the eclipse period passed, after which systems were turned back on and another 6 months of observations were made. TD-1A was primarily a UV mission however it carried both a cosmic X-ray and a gamma-ray detector. TD-1A reentered on Jan 9 1980.

Following on the success of Uhuru (SAS 1), NASA launched the Second Small Astronomy Satellite SAS 2. It was launched from the San Marco platform off the coast of Kenya, Africa, into a nearly equatorial orbit.

To conduct experiments in X-ray astronomy and solar physics among others the Indian Space Research Organization (ISRO) built Aryabhata. It was launched by the Soviet Union on Apr 19 1975 from Kapustin Yar. A power failure halted experiments after 4 days in orbit.

Almost from the beginning of satellite X-ray studies the Soviet Union began placing a large number of solar X-ray satellites into orbit, including those of the Intercosmos series. Many of these were apparently the Vertikal (solar UV/X-ray) satellites: Vertikal-2 (Aug 20 1971), 3 (Sep 2 1975), 5 (Aug 30 1977), 8 (Sep 26 1979), and 9 (Aug 28 1981). Signe 3 (launched on Jun 17 1977) was part of the Intercosmos program. A later satellite Intercosmos 26 (launched on Mar 2 1994) as part of the Coronas-I international project may have conducted X-ray studies of the Sun.

Bhaskara was the second Indian Space Research Organization (ISRO) satellite. It was launched on Jun 7 1979 with a modified SS-5 (SKean IRBM) plus upper stage from Kapustin Yar in the Soviet Union. A secondary objective was to conduct X-ray astronomy investigations. Bhaskara 2 was launched on Nov 20 1981 from Kapustin Yar like its predecessor also in size, mass and design may have conducted X-ray astronomy investigations.

Surveying and cataloging X-ray sources

OSO 7 was primarily a solar observatory designed to point a battery of UV and X-ray telescopes at the Sun from a platform mounted on a cylindrical wheel. The detectors for observing cosmic X-ray sources were X-ray proportional counters. The hard X-ray telescope operated over the energy range 7 - 550 keV. OSO 7 performed an X-ray All-sky survey and discovered the 9-day periodicity in Vela X-1 which led to its optical identification as a HMXRB. OSO 7 was launched on Sep 29 1971 and operated until May 18 1973.

Skylab, a science and engineering laboratory, was launched into Earth orbit by a Saturn V rocket on May 14 1973. Detailed X-ray studies of the Sun were performed. The S150 experiment performed a faint X-ray source survey. The S150 was mounted atop the SIV-B upper stage of the Saturn 1B rocket which orbited briefly behind and below Skylab on Jul 28 1973. The entire SIV-B stage underwent a series of preprogrammed maneuvers, scanning about 1 degree every 15 seconds, to allow the instrument to sweep across selected regions of the sky. The pointing direction was determined during data processing, using the inertial guidance system of the SIV-B stage combined with information from two visible star sensors which formed part of the experiment. Galactic X-ray sources were observed with the S150 experiment. The experiment was designed to detect 4.0-10.0 nm photons. It consisted of a single large (~ 1500 cm) proportional counter, electrically divided by fine wire ground planes into separate signal-collecting areas and looking through collimator vanes. The collimators defined 3 intersecting fields of view (~2 x 20 degrees) on the sky, which allowed source positions to be determined to ~ 30'. The front window of the instrument consisted of a 2 µm thick plastic sheet. The counter gas was a mixture of argon and methane. Analysis of the data from the S150 experiment provided strong evidence that the soft X-ray background cannot be explained as the cumulative effect of many unresolved point sources.

Skylabs solar studies: UV and X-ray solar photography for highly ionized atoms, X-ray spectrography of solar flares and active regions, and X-ray emissions of lower solar corona.

Salyut 4 space station was launched on Dec 26 1974. It was in an orbit of 355 x 343 km, with an orbital period of 91.3 minutes, inclined at 51.6 degrees. The X-ray telescope began observations on Jan 15 1975.

The third US Small Astronomy Satellite (SAS-3) was launched on May 7 1975, with 3 major scientific objectives: 1) determine bright X-ray source locations to an accuracy of 15 arcseconds; 2) study selected sources over the energy range 0.1-55 keV; and 3) continuously search the sky for X-ray novae, flares, and other transient phenomena. It was a spinning satellite with pointing capability. SAS 3 was the first to discover X-rays from an highly magnetic WD binary system, AM Her, discovered X-rays from Algol and HZ 43, and surveyed the soft X-ray background (0.1-0.28 kev).

Orbiting Solar Observatory (OSO 8) was launched on Jun 21 1975. While OSO 8's primary objective was to observe the Sun, four instruments were dedicated to observations of other celestial X-ray sources brighter than a few milliCrab. A sensitivity of 0.001 of the Crab nebula source (= 1 "mCrab"). OSO 8 ceased operations on Oct 1 1978.

X-ray source variability

Although several earlier X-ray observatories initiated the endeavor to study X-ray source variability, once the catalogs of X-ray sources were firmly established, more extensive studies could commence.

Prognoz 6 carried two NaI(Tl) scintillators (2-511 keV, 2.2-98 keV), and a proportional counter (2.2-7 keV) to study solar X-rays.

The Space Test Program spacecraft P78-1 or Solwind was launched on Feb 24 1979 and continued operating until Sep 13 1985, when it was shot down in orbit during an Air Force ASM-135 ASAT test. The platform was of the Orbiting Solar Observatory (OSO) type, with a solar-oriented sail and a rotating wheel section. P78-1 was in a noon-midnight, Sun-synchronous orbit at 600 km altitude. The orbital inclination of 96° implied that a substantial fraction of the orbit was spent at high latitude, where the particle background prevented detector operation. In-flight experience showed that good data were obtained between 35 degrees N and 35 degrees S geomagnetic latitude outside the South Atlantic Anomaly. This yields an instrument duty cycle of 25-30%. Telemetry data were obtained for about 40-50% of the orbits, yielding a net data return of 10-15%. Though this data rate appears low, it means that about 10 seconds of good data reside in the XMON data base.

Data from the P78-1 X-Ray Monitor experiment offered source monitoring with a sensitivity comparable to that of instruments flown on SAS-3, OSO-8, or Hakucho, and the advantages of longer observing times and unique temporal coverage. Five fields of inquiry were particularly well suited for investigation with P78-1 data:

• study of pulsational, eclipse, precession, and intrinsic source variability on time scales of tens of seconds to months in galactic X-ray sources.
• pulse timing studies of neutron stars.
• identification and study of new transient sources.
• observations of X-ray and gamma-ray bursts, and other fast transients.
• simultaneous X-ray coverage of objects observed by other satellites, such as HEAO-2 and 3, as well as bridging the gap in coverage of objects in the observational timeline.

Launched on Feb 21 1981 the Hinotori satellite observations of the 1980s pioneered hard X-ray imaging of solar flares.

Tenma was the second Japanese X-ray astronomy satellite launched on Feb 20 1983. Tenma carried GSFC detectors which had an improved energy resolution (by a factor of 2) compared to proportional counters and performed the first sensitive measurements of the iron spectral region for many astronomical objects. Energy Range: 0.1 keV - 60 keV. Gas Scintillator Proportional Counter: 10 units of 80 cm each, FOV ~ 3deg (FWHM), 2 - 60 keV. Transient Source Monitor: 2 - 10 keV.

The Soviet Astron orbital station was designed primarily for UV and X-ray astrophysical observations. It was injected into orbit on Mar 23 1983. The satellite was put into a highly elliptical orbit, ~200,000 x ~ 2,000 km. The orbit kept the craft far away from the Earth for 3.5 out of every 4 days. It was outside of the Earth's shadow and radiation belts for 90% of the time. The second major experiment, SKR-02M, aboard Astron was an X-ray spectrometer, which consisted of a proportional counter sensitive to 2-25 keV X-rays, with an effective area of 0.17 m. The FOV was 3° x 3° (FWHM). Data could be telemetered in 10 energy channels. The instrument began taking data on Apr 3 1983.

Spacelab 1 was the first Spacelab mission in orbit in the payload bay of the Space Shuttle (STS-9) from Nov 28-Dec 8 1983. An X-ray spectrometer, measuring 2-30 keV photons (although 2-80 keV was possible), was on the pallet. The primary science objective was to study detailed spectral features in cosmic sources and their temporal changes. The instrument was a gas scintillation proportional counter (GSPC) with ~ 180 cm area and energy resolution of 9% at 7 keV. The detector was collimated to a 4.5 deg (FWHM) FOV. There were 512 energy channels.

Spartan 1 was deployed from the Space Shuttle Discovery (STS-51G) on Jun 20 1985 and retrieved 45.5 hours later. The X-ray detectors aboard the Spartan platform were sensitive to the energy range 1-12 keV. The instrument scanned its target with narrowly collimated (5 arcmin x 3 degrees) GSPCs. There were 2 identical sets of counters, each having ~ 660 cm effective area. Counts were accumulated for 0.812 s into 128 energy channels. The energy resolution was 16% at 6 keV. During its 2 days of flight, Spartan-1 observed the Perseus cluster of galaxies and our galactic center region.

Ginga was launched on Feb 5 1987. The primary instrument for observations was the Large Area Proportional Counter (LAC).

The European Retrievable Carrier (EURECA) was launched Jul 31 1992 by the Space Shuttle Atlantis, and put into an orbit at an altitude of 508 km. It began its scientific mission on Aug 7 1992. EURECA was retrieved on Jul 1 1993 by the Space Shuttle Endeavor and returned to Earth. On board was the WATCH or Wide Angle Telescope for Cosmic Hard X-rays instrument. The WATCH instrument was sensitive to 6-150 keV photons. The total field of view covered 1/4 of the celestial sphere. During its 11 month lifetime, EURECA tracked the Sun and WATCH gradually scanned across the entire sky. Some 2 dozen known X-ray sources were monitored - some for more than 100 days. Also, a number of new X-ray transients were discovered.

The Diffuse X-ray Spectrometer (DXS) STS-54 package was flown as an attached payload in Jan 1993 to obtain spectra of the diffuse soft X-ray background. DXS obtained the first-ever high resolution spectra of the diffuse soft X-ray background in the energy band from 0.15 to 0.28 keV (4.3-8.4 nm).

X-1 X-ray sources

As all-sky surveys are performed and analyzed or once the first extrasolar X-ray source in each constellation is confirmed, it is designated X-1, e.g., Scorpius X-1 or Sco X-1. There are 88 official constellations. Often the first X-ray source is a transient.

As X-ray sources have been better located, many of them have been isolated to extragalactic regions such as the Large Magellanic Cloud (LMC). When there are often many individually discernible sources, the first one identified is usually designated as the extragalactic source X-1, e.g., Small Magellanic Cloud (SMC) X-1, a HMXRB, at 011514 -734222.

X-ray source catalogs

Each of the major observatory satellites had its own catalog of detected and observed X-ray sources. These catalogs were often the result of large area sky surveys. Many of the X-ray sources have names that come from a combination of a catalog abbreviation and the Right Ascension (RA) and Declination (Dec) of the object. For example, 4U 0115+63, 4th Uhuru catalog, RA=01 hr 15 min, Dec=+63°; 3S 1820-30 is the SAS-3 catalog; EXO 0748-676 is an Exosat catalog entry; HEAO 1 uses H; Ariel 5 is 3A; Ginga sources are in GS; general X-ray sources are in the X catalog.

The Uhuru X-ray satellite made extensive observations and produced at least 4 catalogs wherein previous catalog designations were improved and relisted: U or 1U 1615+38 would appear successively as 2U 1615+38, 3U 1615+38, and 4U 1615+3802, for example. After over a year of initial operation the first catalog was produced. The fourth and final Uhuru catalog included 339 sources.

Although apparently not containing extrasolar sources from the earlier OSO satellites, the MIT/OSO 7 catalog contains 185 sources from the OSO 7 detectors and sources from the 3U catalog.

The 3rd Ariel 5 SSI Catalog (designated 3A) contains a list of X-ray sources detected by the University of Leicester's Sky Survey Instrument (SSI) on the Ariel 5 satellite. This catalog contains both low and high galactic latitude sources and includes some sources observed by HEAO 1, Einstein, OSO 7, SAS 3, Uhuru, and earlier, mainly rocket, observations.

The 842 sources in the HEAO A-1 X-ray source catalog were detected with the NRL Large Area Sky Survey Experiment on the HEAO 1 satellite.

The Catalog of High-Mass X-ray Binaries in the Galaxy (4th Ed.) contains source name(s), coordinates, finding charts, X-ray luminosities, system parameters, and stellar parameters of the components and other characteristic properties for 114 HMXBs, together with a comprehensive selection of the relevant literature. About 60% of the high-mass X-ray binary candidates are known or suspected Be/X-ray binaries, while 32% are supergiant/X-ray binaries (SGXB).

For all the main-sequence and subgiant stars of spectral types A, F, G, and K and luminosity classes IV and V listed in the Bright Star Catalogue (BSC, also known as the HR Catalogue) that have been detected as X-ray sources in the ROSAT All-Sky Survey (RASS), there is the RASSDWARF - RASS A-K Dwarfs/Subgiants Catalog. The total number of RASS sources amounts to ~150,000 and in the BSC 3054 late-type main-sequence and subgiant stars of which 980 are in the catalog, with a chance coincidence of 2.2% (21.8 of 980).

When EXOSAT was slewing between different pointed observations from 1983 to 1986, it scanned a number of X-ray sources (1210). From this the EXOSAT Medium Energy Slew Survey catalog was created. From the use of the Gas Scintillation Proportional Counter (GSPC) on board EXOSAT, a catalog of iron lines from some 431 sources was made available.

Major questions in X-ray astronomy

Stellar magnetic fields

Magnetic fields are ubiquitous among stars, yet we do not understand precisely why, nor have we fully understood the bewildering variety of plasma physical mechanisms that act in stellar environments. "We have found magnetic fields on stars that ought to have none so long as we appeal to our limited understanding of magnetic field production and amplification. We witness various processes of energy transport and energy release intimately related to those very magnetic fields; the fields not only guide mass and energy flows, they are the sources of energy themselves. But our understanding of energy dissipation has remained patchy, in particular in magnetically active stars."

Extrasolar X-ray source astrometry

With the initial detection of an extrasolar X-ray source, the first question usually asked is "What is the source?" An extensive search is often made in other wavelengths such as visible or radio for possible coincident objects. Many of the verified X-ray locations still do not have readily discernible sources. X-ray astrometry becomes a serious concern that results in ever greater demands for finer angular resolution and spectral radiance.

There are inherent difficulties in making X-ray/optical, X-ray/radio, and X-ray/X-ray identifications based solely on positional coincidents, especially with handicaps in making identifications, such as the large uncertainties in positional determinants made from balloons and rockets, poor source separation in the crowded region toward the galactic center, source variability, and the multiplicity of source nomenclature.

Solar X-ray astronomy

Coronal heating problem

In the area of solar X-ray astronomy, there is the coronal heating problem. The photosphere of the Sun has an effective temperature of 5,570 K yet its corona has an average temperature of 1-2 x 10 K. But, the hottest regions are 8-20 x 10 K. The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere.

It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating. The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events—nanoflares.

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona. In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms.

Coronal mass ejection

A coronal mass ejection (CME) is an ejected plasma consisting primarily of electrons and protons (in addition to small quantities of heavier elements such as helium, oxygen, and iron), plus the entraining coronal closed magnetic field regions. Evolution of these closed magnetic structures in response to various photospheric motions over different time scales (convection, differential rotation, meridional circulation) somehow leads to the CME. Small-scale energetic signatures such as plasma heating (observed as compact soft X-ray brightening) may be indicative of impending CMEs.

The soft X-ray sigmoid (an S-shaped intensity of soft X-rays) is an observational manifestation of the connection between coronal structure and CME production. "Relating the sigmoids at X-ray (and other) wavelengths to magnetic structures and current systems in the solar atmosphere is the key to understanding their relationship to CMEs."

The first detection of a Coronal mass ejection (CME) as such was made on Dec 1 1971 by R. Tousey of the US Naval Research Laboratory using OSO 7. Earlier observations of coronal transients or even phenomena observed visually during solar eclipses are now understood as essentially the same thing.

The largest geomagnetic perturbation, resulting presumably from a "prehistoric" CME, coincided with the first-observed solar flare, in 1859. The flare was observed visually by Richard Christopher Carrington and the geomagnetic storm was observed with the recording magnetograph at Kew Gardens. The same instrument recorded a crotchet, an instantaneous perturbation of the Earth's ionosphere by ionizing soft X-rays. This could not easily be understood at the time because it predated the discovery of X-rays (by Roentgen) and the recognition of the ionosphere (by Kennelly and Heaviside).

Exotic X-ray sources

Microquasar

A microquasar is a smaller cousin of a quasar that is a radio emitting X-ray binary, with an often resolvable pair of radio jets. SS 433 is one of the most exotic star systems observed. It is an eclipsing binary with the primary either a black hole or neutron star and the secondary is a late A-type star. SS 433 lies within SNR W50. The material in the jet traveling from the secondary to the primary does so at 26% of light speed. The spectrum of SS 433 is affected by Doppler shifts and by relativity: when the effects of the Doppler shift are subtracted, there is a residual redshift which corresponds to a velocity of about 12,000 kps. This does not represent an actual velocity of the system away from the Earth; rather, it is due to time dilation, which makes moving clocks appear to stationary observers to be ticking more slowly. In this case, the relativistically moving excited atoms in the jets appear to vibrate more slowly and their radiation thus appears red-shifted.

Be X-ray binaries

LSI+61°303 is a periodic, radio-emitting binary system that is also the gamma-ray source, CG135+01. LSI+61°303 is a variable radio source characterized by periodic, non-thermal radio outbursts with a period of 26.5 d, attributed to the eccentric orbital motion of a compact object, probably a neutron star, around a rapidly rotating B0 Ve star, with a T_eff ~26,000 K and luminosity of ~10 erg s. Photometric observations at optical and infrared wavelengths also show a 26.5 d modulation. Of the 20 or so members of the Be X-ray binary systems, as of 1996, only X Per and LSI+61°303 have X-ray outbursts of much higher luminosity and harder spectrum (kT ~ 10-20 keV) vs. (kT ≤ 1 keV); however, LSI+61°303 further distinguishes itself by its strong, outbursting radio emission. "The radio properties of LSI+61°303 are similar to those of the "standard" high-mass X-ray binaries such as SS 433, Cyg X-3 and Cir X-1."

Supergiant Fast X-ray Transients (SFXTs)

There are a growing number of recurrent X-ray transients, characterized by short outbursts with very fast rise times (~ tens of minutes) and typical durations of a few hours that are associated with OB supergiants and hence define a new class of massive X-ray binaries: Supergiant Fast X-ray Transients (SFXTs). XTE J1739–302 is one of these. Discovered in 1997, remaining active only one day, with an X-ray spectrum well fitted with a thermal bremsstrahlung (temperature of ∼20 keV), resembling the spectral properties of accreting pulsars, it was at first classified as a peculiar Be/X-ray transient with an unusually short outburst. A new burst was observed on Apr 8 2008 with Swift.

Virgo X-1

Observations made by Chandra indicate the presence of loops and rings in the hot X-ray emitting gas that surrounds M87. These loops and rings are generated by variations in the rate at which material is ejected from the supermassive black hole in jets. The distribution of loops suggests that minor eruptions occur every six million years.

One of the rings, caused by a major eruption, is a shock wave 85,000 light-years in diameter around the black hole. Other remarkable features observed include narrow X-ray emitting filaments up to 100,000 light-years long, and a large cavity in the hot gas caused by a major eruption 70 million years ago.

The galaxy also contains a notable active galactic nucleus (AGN) that is a strong source of multiwavelength radiation, particularly radio waves.

Magnetars

A magnetar is a type of neutron star with an extremely powerful magnetic field, the decay of which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma rays. The theory regarding these objects was proposed by Robert Duncan and Christopher Thompson in 1992, but the first recorded burst of gamma rays thought to have been from a magnetar was on Mar 5 1979. These magnetic fields are hundreds of thousands of times stronger than any man-made magnet, and quadrillions of times more powerful than the field surrounding Earth. As of 2003, they are the most magnetic objects ever detected in the universe.

On Mar 5 1979, after dropping probes into the atmosphere of Venus, Venera 11 and Venera 12, while in heliocentric orbits, were hit at 10:51 AM EST by a blast of gamma ray radiation. This contact raised the radiation readings on both the probes Konus experiments from a normal 100 counts per second to over 200,000 counts a second, in only a fraction of a millisecond. This giant flare was detected by numerous spacecraft and with these detections was localized by the interplanetary network to SGR 0526-66 inside the N-49 SNR of the Large Magellanic Cloud. And, Konus detected another source in Mar 1979: SGR 1900+14, located 20,000 light-years away in the constellation Aquila had a long period of low emissions, except the significant burst in 1979, and a couple after.

What is the evolutionary relationship between pulsars and magnetars? Astronomers would like to know if magnetars represent a rare class of pulsars, or if some or all pulsars go through a magnetar phase during their life cycles. NASA’s Rossi X-ray Timing Explorer (RXTE) has revealed that the youngest known pulsing neutron star has thrown a temper tantrum. The collapsed star occasionally unleashes powerful bursts of X-rays, which are forcing astronomers to rethink the life cycle of neutron stars.

"We are watching one type of neutron star literally change into another right before our very eyes. This is a long-sought missing link between different types of pulsars," says Fotis Gavriil of NASA’s Goddard Space Flight Center in Greenbelt, Md., and the University of Maryland, Baltimore.

PSR J1846-0258 is in the constellation Aquila. It had been classed as a normal pulsar because of its fast spin (3.1 s) and pulsar-like spectrum. RXTE caught four magnetar-like X-ray bursts on May 31 2006, and another on Jul 27 2006. Although none of these events lasted longer than 0.14 second, they all packed the wallop of at least 75,000 Suns. "Never before has a regular pulsar been observed to produce magnetar bursts," says Gavriil.

"Young, fast-spinning pulsars were not thought to have enough magnetic energy to generate such powerful bursts," says Marjorie Gonzalez, formerly of McGill University in Montreal, Canada, now based at the University of British Columbia in Vancouver. "Here’s a normal pulsar that’s acting like a magnetar."

The observations from NASA's Chandra X-ray Observatory showed that the object had brightened in X-rays, confirming that the bursts were from the pulsar, and that its spectrum had changed to become more magnetar-like. The fact that PSR J1846’s spin rate is decelerating also means that it has a strong magnetic field braking the rotation. The implied magnetic field is trillions of times stronger than Earth’s field, but it’s 10 to 100 times weaker than a typical magnetar. Victoria Kaspi of McGill University notes, "PSR J1846’s actual magnetic field could be much stronger than the measured amount, suggesting that many young neutron stars classified as pulsars might actually be magnetars in disguise, and that the true strength of their magnetic field only reveals itself over thousands of years as they ramp up in activity."

X-ray dark stars

During the solar cycle, as shown in the sequence of images of the Sun in X-rays, the Sun is almost X-ray dark, almost an X-ray variable.

Supergiants

Betelgeuse, on the other hand, appears to be always X-ray dark. The X-ray flux from the entire stellar surface corresponds to a surface flux limit that ranges from 30-7000 ergs s cm at T=1 MK, to ~1 ergs s cm at higher temperatures, five orders of magnitude below the quiet Sun X-ray surface flux. But, due to instrumental limitations of Chandra the presence of low-level emission on the scale of coronal holes cannot be ruled out. Betelgeuse is a red supergiant, and one of the largest and most luminous stars known in the visible range.

Red giants

Hardly any X-rays are emitted by red giants. The cause of the X-ray deficiency may involve

• a turn-off of the dynamo,
• a suppression by competing wind production, or
• strong attenuation by an overlying thick chromosphere.

Prominent bright red giants include Aldebaran, Arcturus, and Gamma Crucis. There is an apparent X-ray "Dividing Line" in the H-R diagram among the giant stars as they cross from the main sequence to become red giants. Alpha Trianguli Australis (α TrA / α Trianguli Australis) appears to be a Hybrid star (parts of both sides) in the "Dividing Line" of evolutionary transition to red giant. α TrA can serve to test the several Dividing Line models.

Low Density Corona: giant stars cannot have MK hot gas without magnetic confinement, else the gas would expand freely at velocities much higher than observed. The lowering of density to lower X-ray emission is at best a refinement involving the topology of magnetic fields.

Absorption of Coronal Emissions: there is no evidence of increased absorption on red giants as compared to typical F and G giants, attenuation caused by cool winds is not enough to account for the drop in observed X-ray emission, and column density estimates cause attenuations some two orders of magnitude greater than current estimates for actual stars.

Cool Loops: a drop in X-ray emission is affected by coronal temperature drop and has been identified with the Dividing Line; thus, it appears that all the emission on Hydrid stars must be due to flaring loops.

Change in Magnetic Field Topology: the surface filling fractions of active regions decrease across the Dividing Line leading to increased wind flow across the Line; hence, mass loss increases from left to right across the Dividing Line while X-ray emission decreases.

Change in the Magnetic Dynamo: as a star crosses the Dividing Line the magnetic dynamo changes from an α - ω type dynamo (large-scale solar-like dynamo) to a small-scale (non-solar like X-ray emission) dynamo. This also explains the change in magnetic field topology.

Arcturus has very weak C IV emission and X-ray fluxes below ROSAT sensitivity.

Normal A stars

There is a rather abrupt onset of X-ray emission around spectral type A7-F0, with a large range of luminosities developing across spectral class F.

In the few genuine late A- or early F-type coronal emitters, their weak dynamo operation is generally not able to brake the rapidly spinning star considerably during their short lifetime so that these coronae are conspicuous by their severe deficit of X-ray emission compared to chromospheric and transition region fluxes; the latter can be followed up to mid-A type stars at quite high levels. Whether or not these atmospheres are indeed heated acoustically and drive an “expanding”, weak and cool corona or whether they are heated magnetically, the X-ray deficit and the low coronal temperatures clearly attest to the inability of these stars to maintain substantial, hot coronae in any way comparable to cooler active stars, their appreciable chromospheres notwithstanding.

Vega and Altair

Altair is spectral type A7V and Vega is A0V. Altair's total X-ray luminosity is at least an order of magnitude larger than the X-ray luminosity for Vega. Vega is rotating rapidly with a velocity of 274 km/s at the equator. The temperature gradient between the poles (near 10,000 K) and the equator (7,600 K) may also mean Vega has a convection zone around the equator. Altair also rotates rapidly, with a velocity at the equator of around 286 km/s. Altair is also not spherical, but is flattened at the poles due to its high rate of rotation. Altair and Vega have lower temperatures at the equator than the poles. But, in Altair the observed X-rays originate in a corona, heated by magnetic fields produced by a dynamo process similar to the Sun's. Any X-ray emission from A0 stars like Vega is too low to be detectable with the sensitivity of Einstein. Alpha Hydri is spectral class F0V and emits X-rays similar to Altair.

X-ray images of the region containing Vega do show a slight increase in soft X-rays:

optical position of Vega -

• α₁₉₅₀ = 1835.23
• δ₁₉₅₀ = +38°44'.3,

point source position -

• α₁₉₅₀ = 1835.3 ± 0.2
• δ₁₉₅₀ = +38°42' ± 2'.

Vega is the first solitary main-sequence star beyond the Sun known to be an X-ray emitter. To understand what's going on in Vega it is important to realize that the coronal magnetic field plays an integral part in the energy balance of coronal plasma. An extensive convection zone is not required, and any star with magnetic field strengths and geometry similar to the Sun's will possess a corona. Magnetic fields on the order of ~30 gauss have been reported for Vega (~1 gauss for the Sun), so perhaps these substantially higher average field strengths compensate for the expected reduced convective activity, resulting in surface X-ray luminosities comparable to the quiet Sun.

A-type dwarfs

The outer convection zone of early F stars is expected to be very shallow and absent in A-type dwarfs, yet the acoustic flux from the interior reaches a maximum for late A and early F stars provoking investigations of magnetic activity in A-type stars along three principal lines:

• searches for genuine magnetic activity in single, normal A-type and early F stars,
• studies of magnetic activity in chemically peculiar Ap/Bp stars, and
• searches for signatures of magnetic fields in very young, forming A-type stars.

Chemically peculiar Bp/Ap stars

Chemically peculiar stars of spectral type Bp or Ap are appreciable magnetic radio sources, most Bp/Ap stars remain undetected, and of those reported early on as producing X-rays only few of them can be identified as probably single stars. When detected, their X-ray luminosities are quite high and do not follow the systematics of earlier-type stars. Given the strong surface magnetic fields in Bp/Ap stars, the currently favored models involve dipolar magnetospheres either featuring equatorial reconnection zones that heat plasma or winds that are magnetically guided to the equatorial plane where they collide and heat up.

X-ray dark planets

X-ray observations offer the possibility to detect (X-ray dark) planets as they eclipse part of the corona of their parent star while in transit. "Such methods are particularly promising for low-mass stars as a Jupiter-like planet could eclipse a rather significant coronal area."

Single X-ray stars

In addition to the Sun there are many unary stars or star systems throughout the galaxy that emit x-rays. β Hydri (G2 IV) is a normal single, post main-sequence subgiant star, T_eff = 5800 K. It exhibits coronal X-ray fluxes.

The benefit of studying single stars is that it allows measurements free of any effects of a companion or being a part of a multiple star system. Theories or models can be more readily tested. See, e.g., Betelgeuse, Red giants, Vega and Altair, and Capella.

See also

• Ultraviolet astronomy
• Gamma-ray astronomy
• Stellar surface fusion
• Sounding rocket X-ray astronomy
• X-rays from Eridanus
• Stellar X-ray astronomy
• Solar X-ray astronomy
• X-ray telescope
• X-Ray telescopes Alphabetic list

Sources

The content of this article was adapted and expanded from http://imagine.gsfc.nasa.gov/ (Public Domain)

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External links

• X-ray all-sky survey on WIKISKY

v t e Supernovae
Classes	Type Ia Type Iax Type Ib and Ic Type II (IIP, IIL, IIn, and IIb) Hypernova Superluminous Pair-instability Calcium-rich Common envelope jets supernova
Physics of	Carbon detonation Foe/Bethe Near-Earth Phillips relationship Nucleosynthesis p-process r-process γ-process Neutrinos
Related	Failed Fast blue optical transient Fast radio burst Gamma-ray burst Gravitational wave Kilonova Luminous red nova Micronova Nova Pulsar kick Quark-nova Soft gamma repeater Imposter pulsational pair-instability Symbiotic nova
Progenitors	Hypergiant yellow Red Luminous blue variable Supergiant blue red yellow White dwarf Wolf–Rayet star Super-AGB star Population III star
Remnants	Supernova remnant Pulsar wind nebula Neutron star pulsar magnetar Stellar black hole Compact star electroweak exotic quark Zombie star Local Bubble Superbubble Orion–Eridanus
Discovery	Guest star History of supernova observation Timeline of white dwarfs, neutron stars, and supernovae
Lists	Candidates Notable Massive stars Most distant Remnants In fiction
Notable	Barnard's Loop Cassiopeia A SN 1054 Crab Nebula iPTF14hls SN 1000+0216 Tycho's Kepler's SN 1885A SN 1987A SN 1994D SN 185 SN 1006 SN 2003fg Remnant G1.9+0.3 SN 2007bi SN 2011fe SN 2014J SN Refsdal Vela Remnant SN 2006gy ASASSN-15lh SN 2016aps SN 2018cow SN 2022jli SN H0pe
Research	ASAS-SN Calán/Tololo Survey High-Z Supernova Search Team Katzman Automatic Imaging Telescope Monte Agliale Supernovae and Asteroid Survey Nearby Supernova Factory Sloan Supernova Survey Supernova/Acceleration Probe Supernova Cosmology Project SuperNova Early Warning System Supernova Legacy Survey Texas Supernova Search
Category:Supernovae [REDACTED] Commons:Supernovae

v t e Black holes
Outline
Types	BTZ black hole Schwarzschild Rotating Charged Virtual Kugelblitz Supermassive Primordial Direct collapse Rogue Malament–Hogarth spacetime
Size	Micro Extremal Electron Stellar Microquasar Intermediate-mass Supermassive Active galactic nucleus Quasar LQG Blazar OVV
Formation	Stellar evolution Gravitational collapse Neutron star Related links Tolman–Oppenheimer–Volkoff limit White dwarf Related links Supernova Micronova Hypernova Related links Gamma-ray burst Binary black hole Quark star Supermassive star Quasi-star Supermassive dark star X-ray binary
Properties	Astrophysical jet Gravitational singularity Ring singularity Theorems Event horizon Photon sphere Innermost stable circular orbit Ergosphere Penrose process Blandford–Znajek process Accretion disk Hawking radiation Gravitational lens Microlens Bondi accretion M–sigma relation Quasi-periodic oscillation Thermodynamics Bekenstein bound Bousso's holographic bound Immirzi parameter Schwarzschild radius Spaghettification
Issues	Black hole complementarity Information paradox Cosmic censorship ER = EPR Final parsec problem Firewall (physics) Holographic principle No-hair theorem
Metrics	Schwarzschild (Derivation) Kerr Reissner–Nordström Kerr–Newman Hayward
Alternatives	Nonsingular black hole models Black star Dark star Dark-energy star Gravastar Magnetospheric eternally collapsing object Planck star Q star Fuzzball Geon
Analogs	Optical black hole Sonic black hole
Lists	Black holes Most massive Nearest Quasars Microquasars
Related	Outline of black holes Black Hole Initiative Black hole starship Black holes in fiction Big Bang Big Bounce Compact star Exotic star Quark star Preon star Gravitational waves Gamma-ray burst progenitors Gravity well Hypercompact stellar system Membrane paradigm Naked singularity Population III star Supermassive star Quasi-star Supermassive dark star Rossi X-ray Timing Explorer Superluminal motion Timeline of black hole physics White hole Wormhole Tidal disruption event Planet Nine
Notable	Cygnus X-1 XTE J1650-500 XTE J1118+480 A0620-00 Sagittarius A* Centaurus A Phoenix Cluster PKS 1302-102 OJ 287 SDSS J0849+1114 TON 618 MS 0735.6+7421 NeVe 1 Hercules A 3C 273 Q0906+6930 Markarian 501 ULAS J1342+0928 PSO J030947.49+271757.31 AT2018hyz Swift J1644+57 1ES 1927+654
Category [REDACTED] Commons

• X-ray astronomy
• Plasma physics
• Space plasmas
• Sun
• G-type main sequence stars
• Astronomical imaging
• Observational astronomy