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Clusters and groups of galaxies are now recognized to be fundamental constituents of the Universe. According to modern theory, most of the material in the Universe that has collapsed into coherent structures lies in groups of galaxies, while clusters are the only entities in the Universe that should be representative of the Universe as a whole. Thus the study of groups and clusters of galaxies is of tremendous importance. One of the most surprising distinguishing characteristics of these, the largest and most massive objects in the Universe, is that they are filled with hot X-ray-emitting gas whose mass exceeds (often by factors of 3–7) the mass of the galaxies in the groups and clusters. This gas is enriched in heavy elements (e.g., oxygen and iron) and is thus the repository of most of the metals in the Universe.

We briefly review the scientific requirements for spatially resolved X—ray spectroscopy in order to determine the mass of the cluster, its metallicity, its state of evolution and the detailed physics of cooling flows. We discuss the available pre-Rosat and BBXRT observations of Perseus, Coma and Virgo and conclude that there is no simple obvious pattern to the temperature or elemental abundance distribution. We discuss the capability of Rosat for spatially resolved spectroscopy and present preliminary BBXRT results on several clusters. The BBXRT results are in good agreement with simple cooling flow theory, in 3 cases confirm the existence of cold material in the cluster, find only shallow abundance gradients and constrain the mass profile in the central regions of the cluster.

Because Sarazin (1986) has written a very extensive review of the x-ray emission from clusters of galaxies I have allowed myself considerable leeway in writing this review. In particular it has allowed me to be incomplete in my references and to omit certain topics. I have chosen some topics because they have been somewhat ignored in the past literature and others for which both the observational and theoretical situation is unclear. This paper is therefore quite different from the oral talk I gave at Cargese. This review is somewhat different from the usual ones in that I hope that the reader finds the observational situation to be somewhat untidy and not well explained, and that the need for future data and theory is evident

HEAO 1 A2 and Einstein SSS spectral observations of Seyfert galaxies and BL Lac objects suggest that in both cases, the X-ray emission is due to relativistic particles. The five BL Lac objects have very soft spectra and at higher energies (above 10 keV) may have hard tails. Combining our X-ray data with radio, infrared, optical, and ultraviolet observations, we can fit the BL Lac spectra with the familiar synchrotron self-Compton model if we allow for relativistic beaming (Urry and Mushotzky 1982, Urry et al. 1982). We show that Doppler beaming of an underlying (Seyfert-like) source population flattens the observed luminosity function, and we emphasize that the relative numbers of BL Lacs and quasars in given spectral intervals are strong functions of selection effects, the degree of Doppler beaming, and the form of the intrinsic luminosity function.

The November 19, 1978 Gamma-Ray Burst has a very well determined error box, 10 square arcmin (Cline et al., 1981). An 8000 second IPC exposure with the Einstein Observatory detected, at a 3.40 level, one low intensity (≤ 3 10−13 erg/cm2 sec) X-ray source inside the error box. The probabiliy of a “Serendipitous” detection was 0.01. Inside the X-ray source error box there are two weak radio sources, one of them highly polarized (Hjellming and Ewald, 1981) and two mv~ 20 magnitude objects, not coincident with the radio sources, visible in the ESO/SRC J and R plates. With the exception of N49, this is the first possible detection of X-ray emission inside a GRB error box. Its low intensity justifies, in fact, the lack of detection for other events.

The HEAO 1A2 spectra of clusters of galaxies are used to determine the temperature profile which characterizes the X-ray emitting gas. Strong evidence of nonisothermality is found for the Coma, A85, and A1795 clusters. Properties of the cluster potential which binds the gas are calculated for a range of model parameters. The typical binding mass, if the gas is adiabatic, is 2-4E14 solar masses and is quite centrally concentrated. In addition, the Fe abundance in Coma is .26 + or - .06 solar, less than the typical value (.5) found for rich clusters. The results for the gas in Coma may imply a physical description of the cluster which is quite different from what was previously believed.

Clusters of galaxies were established as a class of X-ray sources by observers on the Uhuru satellite (Gursky et al. 1971). These early results indicated that clusters were a powerful (Lx in the 2–6 keV band > 1044 erg/sec) and numerous (≳ 9) class of extra-galactic X-ray sources. These early results also indicated that the sources were extended (Kellogg 1972; Kellogg and Murray 1974) with a characteristic size ≳ .25 Mpc. Early spectral data (Kellogg, Baldwin and Koch 1974; Catura et al. 1972; Gorenstein et al. 1973) showed that the spectra were not strongly cutoff at low energies and could be fit equally well by power laws of energy index ∿ 1 or by the emission from optically thin hot gas (henceforth referred to as thermal bremsstrahlung and abbreviated as T—B) with a temperature of ∿ 108 °K.