|Annu. Rev. Astron. Astrophys. 2000. 38: 289-335
Copyright © 2000 by Annual Reviews. All rights reserved
While there were hints from Einstein observations that some groups of galaxies might contain a hot intragroup medium, it was not until the 1990s that the presence of diffuse gas in groups was firmly established. Group studies were aided by the launch of two important X-ray telescopes, ROSAT (the ROentgen SATellite) and ASCA (Advanced Satellite for Cosmology and Astrophysics). Both of these telescopes were capable of simultaneous X-ray imaging and spectroscopy in the energy range appropriate for poor groups. In addition, the field of view for both telescopes was large enough that nearby groups could effectively be studied.
ROSAT consisted of two telescopes. The X-ray telescope (Aschenbach 1988) was sensitive to photons in the energy range of 0.1-2.4 keV, whereas the Wide Field Camera (Wells et al 1990) covered the energy range 0.070-0.188 keV. The relatively high luminosity of the X-ray background combined with the strong effects of absorption by the Galaxy limited the study of diffuse extragalactic gas with the Wide Field Camera. Therefore, this instrument was not useful for studies of groups and will not be discussed further. Two different kinds of detectors were used with the X-ray telescope: the Position Sensitive Proportional Counter (PSPC) and the High Resolution Imager (HRI). ROSAT was flown with two nearly identical PSPC detectors (Pfeffermann et al 1988). The low internal background, large field of view, and good sensitivity to soft X-rays made the PSPC detectors ideal for studying X-ray emission from groups. The PSPC detectors also had modest energy resolution, allowing the spectral properties of the X-ray emission to be studied. Although the ROSAT HRI provided higher spatial resolution than the PSPC detectors (~ 5" versus ~ 25" for an on-axis source), the internal background of the HRI was high enough that the low surface brightness diffuse emission found in groups could in general not be studied with this instrument. Therefore, most ROSAT studies of groups were performed with the PSPC.
The ROSAT mission consisted of two main scientific phases. The first was a six-month, all-sky survey (Voges 1993) performed with one of the PSPC detectors (until that detector was destroyed during an accidental pointing at the Sun in January 1991). The mean exposure time for the all-sky survey was approximately 400 seconds. Following the completion of the survey, ROSAT was operated in so-called "pointed mode" - that is, with longer pointings at individual targets. Typical exposure times during the pointed mode of the mission were in the range 5000 to 25,000 seconds, or roughly 10 to 50 times longer than the all-sky survey exposures. Although the pointed mode of the ROSAT mission lasted until early 1999, the second PSPC detector ran out of gas in late 1994, effectively ending studies of diffuse emission in groups.