X-Ray Astronomy

I

Introduction

X-Ray Astronomy, study of X-rays arriving at the Earth from space. X-rays are photons of much shorter wavelength than visible and ultraviolet light, and cannot penetrate our atmosphere. They can, therefore, only be studied using X-ray telescopes on rockets or orbiting satellites. These detect the X-ray photons using devices similar to Geiger counters, as well as more modern solid-state detectors, such as charge-coupled devices (CCDs).

II

The Development of X-Ray Astronomy

X-rays can be generated in several ways. Thermal X-rays are emitted by gas at temperatures of millions of degrees—the surface temperature of the Sun, by contrast, is only about 5,800 K. 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. However, 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 marked the beginning of X-ray astronomy as a major field.

By the early 1970s the first all-sky survey had been conducted by the Uhuru satellite, which used conventional Geiger counters. Uhuru mapped out 339 sources, the brightest of which are all within our galaxy. This survey was dramatically extended during the 1990s by the first all-sky imaging survey, carried out by ROSAT, the German/British/United States Röntgen Satellite. 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.

III

X-Ray Observatories

X-rays cannot be focused by “normal” telescope mirrors. If they are to form an image, they must be focused using nested conical mirrors, which the X-rays strike at a small (grazing) angle of incidence. The American Einstein X-ray Observatory, which was launched by NASA and which operated from 1978 to 1981, was the first major telescope to produce true X-ray images. Several such telescopes were operational during the 1990s: ROSAT, the Japanese Advanced Satellite for Cosmology and Astrophysics (ASCA), and the Italian Beppo SAX. But the most spectacular data so far received has been from the Chandra X-ray Observatory launched by NASA in July 1999 and ESA’s XMM-Newton, launched six months later. These two facilities are promising to turn the first decade of the 21st century into a golden age for X-ray astronomy. They have provided detailed images of the X-ray sky, with far greater spatial resolution than previous missions, but more importantly are opening the field of high-resolution cosmic X-ray spectroscopy. As well as detecting a vast number of distant X-ray sources, Chandra has provided detailed images of planetary nebulae, supernovae, and the centres of galaxies showing features such as a bubble of hot gas surrounding a dying star and containing heavy elements from its core, and a jet of high-energy particles streaming away from a supermassive black hole at the centre of an active galaxy.

IV

Objects in the X-Ray Sky

A variety of different kinds of X-ray source can be detected by X-ray observatories.