4.1. Detections and identifications
The first extragalactic object to be detected as an X-ray source was M87 in the Virgo cluster (Byram et al., 1966; Bradt et al., 1967). Sources associated with the Perseus cluster (Fritz et al., 1971; Gursky et al., 1971a) and the Coma cluster (Meekins et al., 1971; Gursky et al., 1971b) were detected next. The idea that extragalactic X-ray sources were generally associated with groups or clusters of galaxies was suggested by Cavaliere et al. (1971). While the early detections were made with balloon- or rocket-borne detectors, a major advance in the study of X-ray clusters (and all of X-ray astronomy) came with the launch of the Uhuru X-ray satellite, which permitted more extended observations of individual sources and a complete survey of the sky in X-rays, the Uhuru Catalog (Giacconi et al., 1972, 1974; Forman et al., 1978a).
The early Uhuru observations established a number of properties of the X-ray sources associated with clusters. First, clusters of galaxies are the most common bright extragalactic X-ray sources. Second, clusters are extremely luminous in their X-ray emission, with luminosities 1043-45 ergs/s, and they have a wide range of luminosities. This makes clusters as a class the most luminous X-ray sources in the universe, with the exception of quasars. Third, the X-ray sources associated with clusters are extended (Kellogg et al., 1972; Forman et al., 1972); the sizes found from the Uhuru data range from about 200 to 3000 kpc. Fourth, the clusters have X-ray spectra that show no strong evidence for low energy photoabsorption, unlike the spectra of the compact sources associated with discrete sources either in the nuclei of galaxies or stellar sources within our own galaxy. Fifth, the X-ray emission from clusters is not time variable, as is the emission from many point sources of X-rays in our galaxy or in the nuclei of other galaxies (Elvis, 1976). These last three results suggest that the emission is truly diffuse, and not the result of one or many compact sources. Many of the early Uhuru results are presented in several review papers by Kellogg (1973, 1974, 1975).
The original identifications of clusters with Uhuru sources were made by Gursky et al. (1972) and Kellogg et al. (1971, 1973). Other identifications of clusters as Uhuru sources were made by Bahcall (1974c), Disney (1974), Rowan-Robinson and Fabian (1975), Elvis et al. (1975), Melnick and Quintana (1975), Vidal (1975a, b), Bahcall et al. (1976), Ives and Sanford (1976), Pye and Cooke (1976), Lugger (1978), and Johnston et al. (1981). The situation on the identifications of clusters was summarized by Bahcall and Bahcall (1975), who argued that the many unidentified Uhuru sources at high galactic latitude were probably also clusters of galaxies. A systematic search for Abell cluster identifications in the Uhuru catalog was made by Kellogg et al. (1973). Cluster X-ray sources were also identified in surveys made with the Ariel 5 satellite (Elvis et al., 1975; Cooke and Maccagni, 1976; Maccacaro et al., 1977; Mitchell et al., 1977; McHardy, 1978a; Ricketts, 1978; McHardy et al., 1981) and SAS-C satellite (Markert et al., 1976, 1979). A combined sample of X-ray identification from the Uhuru and Ariel surveys was given by Jones and Forman (1978), and the pre-HEAO cluster identifications have been reviewed by Gursky and Schwartz (1977).
The next major advance in sensitivity came with the launch of the HEAO-1 X-ray observatory with proportional counters with a very large collecting area. Identifications of clusters with hard X-ray sources detected in sky surveys with the HEAO-1 A-2 instrument have been given by Marshall et al (1979) and by Piccinotti et al. (1982) for high galactic latitude X-ray sources. Searches for X-ray emission from the richest Abell clusters were made by Pravdo et al. (1979). A soft X-ray survey of a few clusters was made by Reichert et al. (1981), and a complete soft X-ray catalog is given by Nugent et al. (1983). A statistically complete sample of Abell clusters was surveyed with the HEAO-1 A-2 instrument by McKee et al. (1980). A survey of a large sample ( 1900) of Abell clusters with the HEAO-1 A-1 was made by Ulmer et al. (1981) and Johnson et al. (1983), while a survey of the most distant Abell clusters with the HEAO-1 A-1 detector (Ulmer et al., 1980b) detected 11 such clusters, suggesting that many of them are extremely luminous. The southern cluster catalog of Duus and Newell (1977) was surveyed for X-ray emission using the HEAO-1 A-1 detector by Kowalski et al. (1984); this paper includes a compilation of all HEAO-1 A-1 cluster detections and limits. Wood et al. (1984) is the complete HEAO-1 A-1 X-ray source catalog.
X-ray astronomy made a quantum leap forward with the launch of the Einstein observatory. This was the first satellite with focusing optics for extrasolar X-ray observing. Because of its focusing capability, the sensitivity of this instrument to small sources was orders of magnitude higher than that for any previous X-ray detector. The two major Einstein surveys of X-ray emission from clusters are Abramopoulos and Ku (1983) and Jones and Forman (1984) (see Table 2 below). Other X-ray cluster detections with Einstein include those of Jones et al. (1979), Henry et al. (1979, 1982), Burns et al. (1981c), Forman et al. (1981), Maccagni and Tarenghi (1981), Perrenod and Henry (1981), White et al. (1981a, b, 1987), Bechtold et al. (1983), Soltan and Henry (1983), and Henry and Lavery (1984).
|(1043 erg/s)||(Mpc)||(1014 m)|
Note. The X-ray luminosities scale as Lx h50-2, the X-ray core radii scale as rx h50-1, and the gas masses scale as mg h50 -5/2. The data are from Jones and Forman (1984) and Abramopoupos and Ku (1983); the entries from Abramopoulos and Ku are marked with an *. The value of Lx is only for gas within 0.5 Mpc of the cluster center, while the gas mass is for gas within 3.0 Mpc of the center. The values of rx and are from fits of the X-ray surface brightness to equation 3.7. In the data from Abramopoulos and Ku, was assumed to be unity, and was not derived from the data. The data from Abramopoulos and Ku were adjusted for the definitions of Lx and mg from Jones and Forman, using the fits given by Abramopoulos and Ku.
| Very uncertain value.|
Compact, poor clusters were detected as X-ray sources by Schwartz et al. (1980a, b) and Kriss et al. (1980, 1981, 1983), and will be discussed in more detail in Section 4.7.
Murray et al. (1978) suggested that a number of sources in the 4U Uhuru catalog were associated intrinsically with superclusters (that is, there was more emission than could be accounted for simply by the sum of the emissions from the number of X-ray clusters one would have expected to find in the supercluster). Kellogg (1978) presented evidence that X-ray clusters were located in superclusters, but did not suggest that there was any intrinsic emission associated with the supercluster itself. Subsequent observations have not confirmed the detections of superclusters as a distinct class of X-ray sources (Ricketts, 1978; Pravdo et al., 1979). Forman et al. (1978b) claimed to have detected very large and luminous haloes of X-ray emission about clusters of galaxies, and argued that the extra emission seen by Murray et al. from superclusters might really be the sum of the haloes of the clusters in the superclusters. In general, subsequent observations have not confirmed the presence of these luminous and extensive haloes (Pravdo et al., 1979; Nulsen et al., 1979; Ulmer et al., 1980a; Nulsen and Fabian, 1980).