X-ray astronomy

X-ray astronomy The observation of X-ray emission from celestial objects, which allows us to study violent and energetic processes in the Universe. The subject was born in 1949, when X-rays from the Sun were discovered by a rocket-borne experiment. The first celestial X-ray source, Scorpius X-1, was detected in 1962, also by a sounding rocket. Rockets and high-altitude balloons during the 1960s revealed other individual sources, the existence of a diffuse X-ray background, and the variability of several objects.

The first dedicated X-ray astronomy satellite was Uhuru, launched in 1970, which carried out the first X-ray survey of the sky. Other satellites with X-ray instruments included the Copernicus satellite and the Astronomical Netherlands Satellite. Balloon and rocket experiments also continued throughout this period. The UK's Ariel 5 and the US SAS-3 satellites launched in 1974 and 1975 extended the catalogues of known X-ray sources and monitored their variability.

In 1977 NASA launched the first of its High Energy Astrophysical Observatories (HEAO). HEAO-1, considerably larger than any previous mission, surveyed the sky with unprecedented sensitivity across a large energy range, from 0.1 keV to 10 MeV. HEAO-2, later renamed the Einstein Observatory, was the first satellite to carry a grazing-incidence telescope and record true X-ray images of the sky. This imaging capability, coupled with the ability to study faint sources, revolutionized X-ray astronomy, putting it on an equal footing with other wavelengths.

Progress continued with European, Japanese, and Russian missions. ESA's Exosat carried imaging and non-imaging experiments in a unique 96-hour elliptical orbit in 1983. This allowed long unbroken viewing of sources. Beginning in 1979, Japan launched three missions—Hakucho, Tenma, and Ginga—carrying experiments of increasing size. In particular, the large array of proportional counters on Ginga improved spectral studies of X-ray sources. In 1987 the Roentgen Observatory was docked to the Mir space station. It was followed by the Granat satellite in 1989.

X-ray astronomy has continued to develop rapidly. Rosat, launched in 1990, performed the first imaging sky survey, detecting more than 60 000 sources, a huge increase on the numbers previously catalogued. The Broad Band X-ray Telescope on the Space Shuttle Astro-1 mission in 1990 and the launch of a fourth Japanese mission, ASCA, improved spectral resolution with the use of CCD detectors. BeppoSAX was launched in 1996. Two major X-ray observatories were launched in 1999: Chandra, concentrating on high-resolution imaging and spectroscopy, and XMM-Newton, which has high-throughput spectroscopy. The Japanese *Suzaku observatory was launched in 2005.