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x-ray astronomy

 
Dictionary: x-ray astronomy
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n.
The branch of astronomy that deals with the origin and nature of emissions from extraterrestrial sources in the x-ray range of electromagnetic radiation rather than in the visible range.


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Britannica Concise Encyclopedia: X-ray astronomy
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Study of astronomical objects and phenomena that emit radiation at X-ray wavelengths. Because Earth's atmosphere absorbs most X-rays, X-ray telescopes and detectors are taken to high altitudes or into space by balloons and spacecraft. In 1949 detectors aboard sounding rockets showed that the Sun gives off X-rays, but it is a weak source; it took 30 more years to clearly detect X-rays from other ordinary stars. Beginning with the Uhuru X-ray satellite (launched 1970), a succession of space observatories carried increasingly sophisticated instruments into Earth orbit. Astronomers discovered that most types of stars emit X-rays but usually as a tiny fraction of their energy output. Supernova remnants are more powerful X-ray sources; the strongest sources known in the Milky Way Galaxy are certain binary stars in which one star is probably a black hole. In addition to myriad point sources, astronomers have found a diffuse background of X-ray radiation emanating from all directions; unlike cosmic background radiation, it appears to have many distant individual sources. The Chandra X-Ray Observatory and XMM-Newton X-ray satellite (both launched 1999) have made numerous discoveries relating to the nature and quantity of black holes in the universe, the evolution of stars and galaxies, and the composition and activity of supernova remnants.

For more information on X-ray astronomy, visit Britannica.com.

Sci-Tech Encyclopedia: X-ray astronomy
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The study of x-ray emission from extrasolar sources, including virtually all types of astronomical objects from stars to galaxies and quasars. The x-ray region of the electromagnetic spectrum extends from wavelengths of about 10 picometers to a few tens of nanometers, with shorter wavelengths corresponding to higher-energy photons (1 nm corresponds to about 1000 eV). X-ray astronomy is traditionally divided into broad bands—soft and hard—depending on the energy of the radiation being studied. Observations in the soft band (below about 10 keV) must be carried out above the atmosphere from rockets or satellites, while hard x-ray observations can be made at high altitudes achievable by balloons. See also Electromagnetic radiation; Rocket astronomy; Satellite astronomy; X-rays.

Most observations have been in the soft band. Sky surveys, particularly the ROSAT all-sky survey, have located tens of thousands of sources at a sensitivity of about 1/100,000 of the strength of the brightest source, Sco X-1. Some of these sources are concentrated along the galactic equator. Such a concentration corresponds to objects in the Milky Way Galaxy, particularly in the disk, which contains most of the galactic stars and the spiral arms. Other x-ray sources are uniformly spread over the sky, associated mainly with extragalactic objects, such as individual galaxies, clusters of galaxies, and quasars. See also Milky Way Galaxy.

Galactic sources

Galactic x-ray sources have been identified with different types of unusual objects. The Crab Nebula, which was the first nonsolar x-ray source to be identified with a specific celestial object, is a supernova remnant left over from the explosive death of a star. It contains a rapidly rotating neutron star at its center as well as a nebula consisting of hot gas and energetic particles. About 10% of the x-ray emission from the Crab is pulsed radiation from the neutron star; the rest is extended emission associated with the nebula. Many other galactic supernova remnants have been detected as x-ray sources, and there have also been detections of supernova remnants in the nearest neighboring galaxies, especially in the Large Magellanic Cloud. See also Crab Nebula; Neutron star; Pulsar; Supernova.

While normal stars radiate some energy in the x-ray band, most of their luminosity is output in the visible region of the spectrum. X-ray surveys have discovered a class of objects where the majority of energy is radiated in the x-ray portion of the electromagnetic spectrum. These are known as x-ray stars. With accurate locations, it has been possible to identify them with binary star systems. The combination of x-ray and optical data leads to the conclusion that these systems usually consist of a relatively normal visible star and one subluminous star in a gravitationally bound orbit about each other. The x-ray star is often a collapsed object, such as a white dwarf, neutron star, or even a black hole. Such compact sources form the majority of galactic emitters.

The theoretical model for x-ray emission in these systems consists of matter being transferred from the normal star to the compact object. This process, known as accretion, usually leads to the creation of a disk of infalling material spiraling down toward the surface of the compact star. During infall the matter reaches very high temperatures as gravitational energy is converted to heat, which is then radiated as x-rays. It is believed that similar processes are involved regardless of whether the compact object is a white dwarf, neutron star, or black hole. However, different types of detailed behavior are expected for each of these objects. Rotating neutron stars lead to x-ray pulsars. For white dwarfs, there appear to be fluctuations in x-ray intensity on time scales of hours to days that might be indicative of changes at the surface of the star where accreted material is collecting, and then flashing in a burst of thermonuclear energy release. In the case of black hole candidates, there are variations in x-ray intensity at extremely short time scales that indicate very small regions of x-ray emission.

Another type of time behavior is represented by the so-called bursters. These sources emit at some constant level, with occasional short bursts or flares of increased brightness. These outbursts may well be instabilities in the x-ray emission processes associated with this type of source. See also Binary star; Black hole; Cataclysmic variable; Stellar evolution; White dwarf star.

Extragalactic sources

Among the more interesting types of objects detected have been apparent normal galaxies, galaxies with active nuclei, radio galaxies, clusters of galaxies, and quasars.

In active nuclei galaxies (those with strong optical emission lines and nonthermal continuum spectra), x-ray emission is usually orders of magnitude in excess of that from normal galaxies. In most cases the emission comes from the galactic nucleus, and must be confined to a relatively small region on the basis of variability and lack of structure at the current observational limits of a few seconds of arc. Many astrophysicists believe that galactic nuclei are the sites of massive black holes, at least 106–109 times the mass of the Sun, and that the radiation from these objects is due to gravitational energy released by infalling material. The broad range of properties, such as x-ray luminosity, may reflect the size of the black hole and availability of infalling matter. Quasars may represent the extreme case of this mechanism. Many of the observable properties depend on the viewing angle of the observer as well as the size of the nuclear black hole. See also Quasar.

Clusters of galaxies are collections of hundreds to thousands of individual galaxies which form a gravitationally bound system. The space between galaxies in such clusters has been found to contain hot (approximately 107–108 K) tenuous gas which glows in x-rays, and whose mass is equal to (or exceeds) the mass of the visible galaxies. See also Astrophysics, high-energy; Galaxy, external.

 
Columbia Encyclopedia: X-ray astronomy
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X-ray astronomy, study of celestial objects by means of the X rays they emit, in the wavelength range from 0.01 to 10 nanometers. X-ray astronomy dates to 1949 with the discovery that the sun emits X rays. Since X rays could not be observed from ground-based telescopes, V-2 rockets launched from White Sands, N.Mex., occasionally carried telescopes to study solar X-ray emissions. In 1962 a group led by R. Giacconi launched a small rocket from White Sands to search for celestial sources of X rays with instruments similar to Geiger counters. During the 5-min flight the experiment discovered an X-ray source now called Scorpius X-1, a close binary star in which one star expels gas onto a very dense neighbor, which may be a white dwarf, a neutron star, or a black hole. This mission also found that the earth is bathed in diffuse X rays coming from all directions. Soon afterward X-ray emissions were found coming from the Crab Nebula and the radio galaxies (galaxies whose radio emissions constitute an extraordinarily large amount of their total energy output) Centaurus A and Virgo A. Other types of galaxies, particularly Seyfert galaxies (galaxies with extremely bright cores that are strong emitters of radio waves, X rays, and gamma rays), also emit X rays. The center of our galaxy is a strong X-ray source, which is an indicator of the violent activity taking place there.

In 1970 the Uhuru satellite, one of NASA's small astronomy satellites, began to look specifically for X-ray sources. Uhuru used detectors filled with argon, in which incoming X radiation gives off electrons in amounts proportional to its strength. Uhuru mapped more than 400 sources and discovered a series of X-ray binary stars in which ordinary stars orbit neutron stars that emit X rays. One of these sources, Cygnus X-1, is an object with ten times the mass of the sun. Too massive to be a neutron star, it is possibly a black hole.

Much of the data in X-ray astronomy is now gathered by orbiting satellites. In addition to the United States, Germany and Japan are among the countries having X-ray satellites. In the 1970s the Skylab space station and Orbiting Solar Observatory satellites continued the study, as did the Solar Maximum Mission the following decade. A series of High Energy Astrophysical Observatories (HEAO) were launched during the late 1970s to study X rays, cosmic rays, and gamma rays. HEAO-1, launched in 1977, increased the number of known X-ray sources from 350 to 1,500. HEAO-2-also known as the Einstein Observatory-carried the largest X-ray telescope ever built. It detected several thousand new X-ray sources in our galaxy and beyond, discovered that cataclysmic variable stars in our own galaxy emit X rays when they are in outburst, achieved the first unambiguous detection of X rays from ordinary stars other than the sun, and obtained the first X-ray images of supernova remnants, pulsars, and star clusters. As a result, supernova remnants mapped in X-ray wavelengths can be compared with visible light and radio images. In an example of cooperation between amateur and professional astronomers, the Einstein Observatory was turned toward SS Cygni (see variable star) whenever amateur astronomers with backyard telescopes reported it in outburst. The few days' duration of these outbursts allowed enough time to change the satellite's observing schedule so that it could examine the star, and it discovered the source of the star's X-ray emissions.

During the 1980s the European, Russian, and Japanese space agencies continued to launch successful X-ray astronomy missions, such as the European X-ray Observatory Satellite (EXOSAT), Granat, the Kvant module (of the Mir space station), Tenma, and Ginga. These missions were more modest in scale than the HEAO program in the 1970s and were directed toward in-depth studies of known phenomena.

In 1990, ROSAT [Roentgen Satellite], a joint project of Germany, the United States, and Great Britain, was launched. Operational until 1999, it was instrumental in the discovery of X-ray emissions from comets and conducted an all-sky survey in the X-ray region of the spectrum. Five other satellites launched in the 1990s are still operational. ALEXIS [Array of Low Energy X-ray Imaging Sensors] was launched in 1993; a minisatellite containing six coffee-can-sized wide-angle, ultrasoft-X-ray telescopes, it provided the data for a unique sky map for studying celestial flashes of soft X rays. Also launched in 1993, the Advanced Satellite for Cosmology and Astrophysics is a joint Japanese-American project; containing four X-ray telescopes, its primary purpose is the X-ray spectroscopy of such astrophysical entities as quasars and cosmic background X radiation. In 1995, NASA orbited the Rossi X-ray Timing Explorer (RXTE) to study the variations in the emission of such X-ray sources as black-hole candidates, active galactic nuclei, white dwarf stars, neutron stars, and other high-energy sources. The RXTE played a key role in the discovery in 1996 of a "pulsing burster" located near the center of the Milky Way. Unlike other X-ray sources, this one burst, oscillated, and flickered simultaneously, with bursts lasting from 6 to 100 seconds. Before it burned out, the unexplained object was the brightest source of X rays and gamma rays in the sky, radiating more energy in 10 seconds than the sun does in 24 hours. BeppoSAX, a joint Italian-Dutch satellite, was launched in 1996. When on Dec. 14, 1997, for 1 or 2 seconds the most energetic burst of gamma radiation ever detected was recorded by the Compton Gamma Ray Observatory, BeppoSAX recorded the X-ray afterglow of the burst, thereby providing a relatively accurate location for the source. The Chandra X-ray Observatory was deployed from a shuttle and boosted into a high earth orbit in 1999; it focuses on such objects as black holes, quasars, and high-temperature gases throughout the X-ray portion of the electromagnetic spectrum. Also launched in 1999 was X-ray Multimirror Mission, an ESA satellite that carries an optical-ultraviolet telescope together with three parallel mounted X-ray telescopes, allowing it to simultaneously observe phenomena in two regions of the spectrum.

Wikipedia: X-ray astronomy
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X-rays start at ~0.008 nm and extend across the electromagnetic spectrum to ~8 nm, over which Earth's atmosphere is opaque.

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.

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Featurette

The GOES 14 spacecraft carries a Solar X-ray Imager to monitor the Sun's X-rays for the early detection of solar flares, coronal mass ejections (CME), and other phenomena that impact the geospace environment.

GOES 14 was launched into orbit on June 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.

Sounding rocket flights

See also: Sounding rocket X-ray astronomy
The four-stage Black Brant XII sounding rocket can carry scientific payloads to altitudes from 48 to above 1,287 km. NASA launches an average of 30 sounding rockets each year with a success rate of about 98%.

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 January 28, 1949. X-rays from the Sun were detected by the USA Naval Research Laboratory Blossom experiment on board.[1] An Aerobee 150 rocket launched on June 12, 1962 detected the first X-rays from other celestial sources (Scorpius X-1).[2] 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 106 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 300 Å when used at normal incidence. For instance, at 173 Å 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 km 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 July 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.[3]

High-resolution gamma-ray and hard X-ray spectrometer (HIREGS)

HIREGS attached to launch vehicle while balloon is inflated (1993)

One of the recent balloon-borne experiments was called the High-resolution gamma-ray and hard X-ray spectrometer (HIREGS).[4] 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

The Crab Nebula is a remnant of an exploded star. This is the Crab Nebula in various energy bands, including a hard X-ray image from the HEFT data taken during its 2005 observation run. Each image is 6′ wide.

The high-energy focusing telescope (HEFT) is a balloon-borne experiment to image astrophysical sources in the hard X-ray (20-100 keV) band.[5] 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 March 1, 1949.[1]

A Navy Deacon rockoon just after a shipboard launch, July 1956. The Deacon rocket is suspended below the balloon.

From July 17 to July 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.[6]

X-ray astronomy satellites

Main article: X-ray astronomy satellites

X-ray astronomy satellites study X-ray emissions from celestial objects. Satellites, which can detect and transmit data about the X-ray emissions are deployed as part of branch of space science known as X-ray astronomy. Satellites are needed because 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 telescopes and mirrors

Main article: X-ray telescope
The Swift XRT contains a grazing incidence Wolter I telescope to focus X-rays onto a state-of-the-art CCD. The complete mirror module for the XRT consists of the X-ray mirrors, thermal baffle, a mirror collar, and an electron deflector. To prevent on-orbit degradation of the mirror module's performance, it is be maintained at 20 ± 5 °C, with gradients of <1 °C by an actively controlled thermal baffle (purple, in schematic below) similar to the one used for JET-X. A composite telescope tube holds the focal plane camera (red), containing a single CCD-22 detector.

X-ray telescopes (XRTs) have varying directionality or imaging ability based on glancing angle reflection rather than refraction or large deviation reflection.[7][8] This limits them to much narrow fields of view than visible or UV telescopes. The mirrors can be made of ceramic or metal foil.[9]

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.

X-ray astronomy detectors

Main article: X-ray astronomy detectors

X-ray astronomy 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.

This is an image of the instrument called the Proportional Counter Array on the Rossi X-ray Timing Explorer (RXTE) satellite.

X-ray detectors collect individual X-rays (photons of X-ray electromagnetic radiation) including the number of photons collected (intensity), the energy (0.12 to 120 keV) of the photons collected, wavelength (~0.008 to 8 nm), or how fast the photons are detected (counts per hr), to tell us about the object that is emitting them.

Astrophysical sources of X-rays

Main article: Astrophysical X-ray source

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.[10]

This light curve of Her X-1 shows long term and medium term variability. Each pair of vertical lines delineate the eclipse of the compact object behind its companion star. In this case, the companion is a 2 Solar-mass star with a radius of nearly 4 times that of our Sun. This eclipse shows us the orbital period of the system, 1.7 days.

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.[11]

Hercules X-1 is composed of a neutron star accreting matter from a normal star (HZ Her) probably due to Roche lobe overflow. 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.[12]

Celestial X-ray sources

Main article: Astrophysical X-ray source

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.

This ROSAT PSPC false-color image is of a portion of a nearby stellar wind superbubble (the Orion-Eridanus Bubble) stretching across Eridanus and Orion. Soft X-rays are emitted by hot gas (T ~ 2-3 MK) in the interior of the superbubble. This bright object forms the background for the "shadow" of a filament of gas and dust. The filament is shown by the overlaid contours, which represent 100 micron emission from dust at a temperature of about 30 K as measured by IRAS. Here the filament absorbs soft X-rays between 100 and 300 eV, indicating that the hot gas is located behind the filament. This filament may be part of a shell of neutral gas that surrounds the hot bubble. Its interior is energized by UV light and stellar winds from hot stars in the Orion OB1 association. These stars energize a superbubble about 1200 lys across which is observed in the optical (Hα) and X-ray portions of the spectrum.

Within the constellations Orion and Eridanus and stretching across them is a soft X-ray "hot spot" known as the Orion-Eridanus Superbubble, the Eridanus Soft X-ray Enhancement, or simply the Eridanus Bubble, a 25° area of interlocking arcs of Hα emitting filaments.

Proposed (future) X-ray observatory satellites

Main article: X-ray astronomy satellites

There are several projects that are proposed for X-ray observatory satellites. See main article link above.

Explorational X-ray astronomy

Main article: Explorational X-ray astronomy
Ulysses' second orbit: it arrived at Jupiter February 8, 1992 for a swing-by maneuver that increased its inclination to the ecliptic by 80.2 degrees.

Usually observational astronomy is considered to occur on Earth's surface (or beneath it in neutrino astronomy). The idea of limiting observation to Earth includes orbiting the Earth. As soon as the observer leaves the cozy confines of Earth, the observer becomes a deep space explorer.[13] Except for Explorer 1 and Explorer 3 and the earlier satellites in the series,[14] usually if it's going to be a deep space explorer it leaves the Earth or an orbit around the Earth.

For a satellite or space probe to qualify as a deep space X-ray astronomer/explorer or "astronobot"/explorer, all it needs to carry aboard is an XRT or X-ray detector and leave Earth orbit.

Ulysses was launched October 6, 1990, and reached Jupiter for its "gravitational slingshot" in February 1992. It passed the south solar pole in June 1994 and crossed the ecliptic equator in February 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 23-mm thick × 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 ~10−6 erg/cm2. 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 2.5-mm thick × 0.5 cm2 area Si surface barrier detectors. A 100 mg/cm2 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 °C. This detector had 6 energy channels, covering the range 5-20 keV.

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.[15]

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.[10] 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.[10] 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.[10]

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.[10] Its overall X-ray luminosity rivals the optical emission and could be that of a nonthermal plasma. However, 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.[10]

The "Dividing Line" as giant stars evolve to become red giants also coincides with the Wind and Coronal Dividing Lines.[16] 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.[16]

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 1036 erg s−1.

The mechanism triggering the different temporal behavior observed between the classical SGXBs and the recently discovered SFXTs is still debated.[17]

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.[17] 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.[17] The large variations in the X-ray flux can be explained through accretion of macro-clumps formed within the stellar wind.[17]

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.[17]

Stellar X-ray astronomy

Main article: Stellar X-ray astronomy

Stellar X-ray astronomy started on April 5, 1974 with the detection of X-rays from Capella.[18] 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.[18] The X-ray luminosity of ~1031 erg s−1 is four orders of magnitude above the Sun's X-ray luminosity.[18]

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.[19] 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.[19] 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.[20]

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.[21] 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.[20]

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.[22] 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.[22] The ROSAT All-Sky survey identified tens of thousands of coronal sources.[22] 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.[20]

To fit the medium-resolution spectrum of UX Ari, subsolar abundances were required.[20]

X-ray activity in solar-like main sequence stars is strongly correlated with the period of stellar rotation.[22] 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.[20]

Coronae are ubiquitous among the stars in the cool half of the Hertzsprung-Russell diagram.[20] 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.[20]

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.[20]

Unstable winds

Given the lack of a significant outer convection zone, theory predicts the absence of a magnetic dynamo in earlier A stars.[20] In early stars of spectral type O and B, shocks developing in unstable winds are the likely source of X-rays.[20]

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.[20] As a distributed (or α2) 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.[20] However, 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 (LX) ~1026 erg s−1 and flares up to an order of magnitude higher.[20] Comparison with other late M dwarfs shows a rather continuous trend.[20]

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 10−7.[20] 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 LX.[20]

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

  • reminiscent of hot stars,
  • others point to coronal activity as in cool stars, in particular the presence of flares and very high temperatures.[20]

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.[20]

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 (LX ≥ 1032 erg s−1) and the hottest known with dominant temperatures up to 40 MK.[20] However, 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.[20]

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.[23]

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.[24] 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.[25]

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

CSXT/GoFast sounding rocket launch, May 17, 2004.
See also: Amateur rocketry

The Reaction Research Society on November 23, 1996 launched a solid fueled rocket, designed by longtime member George Garboden, to an altitude of 80 km (50 miles) from Black Rock Desert in Nevada.[26]

On May 17, 2004 Civilian Space eXploration Team (CSXT) successfully launched the first amateur high-power rocket into space, achieving an altitude of 115 km (72 miles).[27]

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.[28] For normal incidence or glancing angle incidence, the main cost limitation is the telescope X-ray optics.

History of X-ray astronomy

Main article: 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."[29]

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.[20] The Sun has been known to be surrounded by a hot tenuous corona.[30] In the mid-1940s radio observations revealed a radio corona around the Sun.[20]

The beginning of the search for X-ray sources from above the Earth's atmosphere was on August 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.[31]

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.

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.[20]

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.[32]

Solar X-ray astronomy

Main article: 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[33] yet its corona has an average temperature of 1-2 × 106 K.[34] However, the hottest regions are 8-20 × 106 K.[34] The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere.[35]

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.[34] The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone.[34] These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat.[36] 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.[37]

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.[38] In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms.[34]

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.[39] 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.[39] "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."[39]

The first detection of a Coronal mass ejection (CME) as such was made on December 1, 1971 by R. Tousey of the US Naval Research Laboratory using OSO 7.[40] 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

Main article: Astrophysical X-ray source
See also: Be X-ray binaries

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. LSI+61°303 is a periodic, radio-emitting binary system that is also the gamma-ray source, CG135+01. 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). Observations made by Chandra indicate the presence of loops and rings in the hot X-ray emitting gas that surrounds Messier 87. 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.

X-ray dark stars

Main article: Astrophysical X-ray source
See also: Supergiant

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. Betelgeuse, on the other hand, appears to be always X-ray dark. Hardly any X-rays are emitted by red giants. 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. 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. 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. 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. 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, Teff = 5800 K. It exhibits coronal X-ray fluxes.[41]

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.

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