X-Ray Studies of Clusters of Galaxies

Clusters of galaxies are next to quasars, the most luminous X-ray sources in the Universe with radiation powers of the order of 10{su43}-10{su46} erg s{su-1}. The first detection of a cluster source was made with M87 in 1966 by Byram et al. [30], and 5 years later also the massive nearby clusters in the constellations Coma Berenices and Perseus were detected by Gursky et al. [71] and Fritz et al. [65]. With the use of the Uhuru satellite, the extended nature of the cluster X-ray sources could be established [85]. It turns out that the diffuse X-ray emission from clusters originates in a hot intracluster plasma with temperatures of several ten Million degrees, which radiates the bulk of its thermal radiation in the soft X-ray regime. As the hot plasma is tracing the shape of the cluster, the X-ray appearance provides us with information on the cluster structure. The soft X-ray band in which clusters radiate is fortunately also the wavelength regime for which X-ray telescopes with imaging optics provide us with a detailed picture of the X-ray sky. Therefore, galaxy clusters are among the most rewarding and informative objects for X-ray imaging studies.