X-Ray Astronomy 

Introduction

The first X-Ray detections (apart from the Sun) were made in in 1962. On June 18, 1962, a bright X-ray source in space was accidentally discovered by a rocket launched by an American group that was attempting to study solar X-rays reflected from the surface of the Moon. This original X-ray source was in the constellation of Scorpius (Scorpius X-1). This marked the beginning of X-ray astronomy as a major field, although the actual identification of Scorpius X-1 had to wait several years. The first cosmic X-ray source to actually be identified was the Crab Pulsar in 1964.

Generation of X-Rays

X-rays can be generated in different ways.

• Thermal X-rays are emitted by gas at temperatures of millions of degrees.

• Very energetic electrons spiralling in powerful magnetic fields produce so-called synchrotron radiation. Few sources with such properties were known before the first X-ray detectors went beyond the atmosphere, and copious X-rays from space were not anticipated.

Fields of Research

Various objects can be studied

• Hot paches and cool 'holes' in the corona of the Sun 

• Supernova Remnants  When a star goes supernova, the blast wave expands into space at thousands of kilometers per second. This blast wave shocks the surrounding interstellar matter and heats it to tens of millions of degrees, producing copious thermal X-rays.

• X-Ray Binaries   The original X-ray source, Scorpius X-1, proved to be associated with a previously unknown 13th-magnitude blue variable star. Subsequently it was shown to be a binary system. It consists of a 'normal' star orbiting a compact object that is thought to be a neutron star. Because of its angular momentum, the transferred material forms a disk of gas about the compact object. Matter from the inner part of the disc falls on to the compact object, liberating huge amounts of energy as it does so. The gas reaches infall velocities of significant fractions of the speed of light and is heated to very high temperatures, hence producing substantial amounts of X-radiation.

  Neutron stars in X-ray binaries can be recognized by their highly periodic signal. These occur preferentially in binaries where the mass-losing star is hot and massive (10 to 20 times the mass of the Sun), indicating that they were formed only a few million years ago, because massive stars are short-lived.

  Giant X-ray bursts, rising in about a second, then decaying in 10 to 30 seconds, are seen from older X-ray binaries, where the mass-losing star is cool and of low mass. These bursts are due to the thermonuclear “runaway” of helium-burning on the surface of the neutron star. The helium is formed from the “steady” burning of the hydrogen which is accreted from the normal star via the accretion disc.

  Dynamical studies at optical wavelengths of the low-mass companion stars in these X-ray binaries show that some of the compact objects exceed six solar masses. Theoretically, neutron stars cannot exceed three solar masses, so these massive compact objects appear to be black holes.

  Binaries involving White Dwarfs will also emit X-rays.

• Quasars   A similar accretion process on to a compact object (see immediately above), the most efficient mechanism known for converting mass into energy, is also believed to account for the extraordinarily high luminosities of quasars. These aree compact objects having masses millions of times that of the Sun. By far the greater number of faint X-ray sources seen by the Einstein satellite and by ROSAT are quasars. Their study at even greater sensitivity by Chandra and XMM has shown that they are responsible for the diffuse X-ray background radiation.

• Intergalactic Hot Gas   Clusters of galaxies represent X-ray emission on the largest scale in the universe. Gas in the space between the galaxies is at an extremely low density - a density lower than the best vacuum obtainable in Earth-based laboratories. This matter is heated to temperatures approaching 50 million K by the motions of the galaxies within the cluster. Clusters are bright X-ray sources because of their truly enormous volume. In 1998 astronomers using ROSAT announced that their observation of the pattern of temperature distribution in these clouds of intergalactic gas demonstrated that the galaxies themselves must have formed before they began grouping into clusters and larger structures. This was a major advance in understanding the process of large-scale evolution in galaxies and galaxy clusters.

• The Diffuse X-Ray Background   The X-ray sky shows one remarkable feature that is very different from the visual night-time sky. There is a diffuse X-ray glow across the entire sky that at higher X-ray energies is remarkably uniform. The origin of this diffuse X-ray background is still not completely understood, but it is believed to be due to the most distant (and hence very young) active galaxies and quasars. At lower X-ray energies (cooler gas) the diffuse background is highly structured and much brighter towards the plane of our galaxy. It is due to old supernova remnants and hot “bubbles” of interstellar gas that are relatively close to us. There is evidence that the solar system lies inside one of these bubbles, which could be explained if a supernova had occurred nearby within the last 10 million years. Recent results from Chandra have pinpointed a huge number of X-ray sources by continuous observation of one patch of sky; these would appear to account for virtually all the X-ray background radiation, and to confirm that it arises from distant galaxies. Many of these galaxies have their core regions obscured from direct view in the optical, but hard X-rays can penetrate the obscuration, thereby accounting for one of the enigmatic features of the X-ray background—its extremely high temperature.

X-Ray Telescopes

• Uhuru The first X-ray satellite, launched in December 1970. It was launched from Kenya to achieve the desired equatorial orbit, thus the name. In the early 1970s, it carried out the first all-sky survey using conventional Geiger counters. Uhuru mapped out 339 sources, the brightest of which are all within our galaxy. Notably, it found that binaries containing a compact object were an important class of X-ray objects.

• Einstein  (America), which operated from 1978 to 1981, was the first major telescope to produce true X-ray images. It found that most of the X-ray background came from quasars.

• ROSAT  (Röntgensatellit) Launched in June 1990, and operated by Deutschland, USA and Britain. The Uhuru survey was dramatically extended during the 1990s by the first all-sky imaging survey, carried out by ROSAT. This located thousands of sources, most of which are very faint and beyond our galaxy, being associated with quasars, active galaxies, and clusters of galaxies.

• Exosat  European. Launched in 1983.

• Chandra (CXO)  America. Named after the Indian-American astrophysicist Subrahmanyan Chandrasekhar, it was launched by NASA on July 23, 1999. CXO was then injected into a high Earth orbit with a period of 63.5 hr, which allows for long, unbroken observations of X-ray sources (compared to the more usual 100 min low Earth orbit, of which typically only about 45 per cent is useable). The observatory, which can produce X-ray images up to 10 times sharper than those obtained by any previous X-ray telescope, also has the capability to undertake (together with the XMM observatory of the European Space Agency), for the first time on faint X-ray sources, high-resolution X-ray spectroscopy. This allows CXO to determine both the composition and dynamics of the X-ray emitting gas (which must be at temperatures of millions of degrees). It has been able to account for the previously puzzling glow of diffuse X-rays that pervade the universe (the so-called “X-ray background”) by resolving this glow into a huge number of faint, individual X-ray sources, most of them galaxies probably containing accreting, massive black holes, some of these being the farthest objects so far detected. CXO has also made studies of the black hole at the centre of the neighbouring Andromeda galaxy, and produced images of cannibalistic galaxies, X-ray jets emanating from the supermassive black holes at the centres of active galaxies (such as M87) and quasars, and glowing planetary nebulae

• XXM-Newton  European.

X-Ray Objects

• Scorpius X-1 The first known source and still the brightest. In 1967, it was identified with a neutron star surrounded by an accretion disk. The accretion disk is now believed to come from a companion binary star, orbiting around the neutron star every 0.78 days.

• Cygnus X-1 First detected by Uhuru. Coincides withn a blue supergiant which appears to orbit a companion in 5.6 days. The mass of this companion is calculated to be between 8 and 16 Solar Masses. This is too massive for a Neutron Star, leading to the conclusion that the companion is a Black Hole.