ROSAT's view of the X-ray sky

Abstract

All-sky X-ray surveys with satellites started in the early 1970s with relatively crude, non-imaging instrumentation. The resulting survey catalogs, for example those from Uhuru & Ariel 5, contained only a few hundred sources, rising only modestly to ∼1000 entries by the end of the decade with the HEAO-1 (A-1) survey. Since then the most important and influential X-ray sky survey has undoubtedly been that carried out by ROSAT in the early 1990s. The ROSAT Bright Source Catalogue paper by Voges et al., when it was published 10 years ago, thus represented not only the culmination of the ROSAT project’s primary aim of surveying the whole sky at X-ray wavelengths with an unprecedented sensitivity, but also a major step forward in our knowledge of the X-ray sky. ROSAT (ROntgen SATellit; Trumper 1982) was a German national project with an international partnership with the UK and US. From its origins as a mission concept in the middle of the 1970s to its realization as ROSAT, the main aim of the project was to perform the first all-sky survey with an imaging X-ray telescope, but the scope of the project was expanded through the international partnerships to encompass a program of pointed observations (with the provision of an additional focal plane detector by the US) and to include an extreme ultraviolet (EUV) capability with a separate EUV telescope supplied by the UK. Originally planned as a space shuttle launch, the Challenger tragedy in 1986 delayed the project until an alternative launch – by a Delta II rocket – could be secured. ROSAT was eventually launched on 1990 June 1, just over a month after the Hubble Space Telescope. ROSAT’s main X-ray telescope operated in the soft X-ray band (0.1–2.4 keV) with two different, interchangeable, detectors in the focal plane: two Position Sensitive Proportional Counters (PSPCs), used for the all-sky survey, and the High Resolution Imager (HRI), provided by the US and used only for pointed observations. The ROSAT EUV telescope had a separate micro-channel plate detector, the ROSAT Wide Field Camera (WFC). The ROSAT All-Sky Survey (RASS) took place in late 1990/early 19911. Rather remarkably, in 6 months observing time, ROSAT surveyed essentially the whole sky to a sensitivity a factor 20 better than previous missions, while simultaneously