Hard X-ray observations of Sco X-1 and GX 1+4

Extra-solar X-ray astronomy started in 1962 with the detection of Sco X-1 and the galactic center during a rocket flight designed to find lunar X-rays. Since then, mainly as a result of a series of dedicated satellite missions the number of known (until 1984) X-ray sources had increased to about 1000. This number is comparable to the number stars brighter than 4th magnitude. The X-ray sky at this sensitivity appears as crowded as a moonlit night. The most comprehensive catalogue is that from the HEAO-1 satellite (1977/1978). This catalogue (Wood et al. 1984) lists 842 sources at energies below 20 keV. Unlike optical wavelengths about half the brightest soft to medium energy X-ray sources are associated with extra-galactic objects which are either galaxy groups or active galactic nuclei (AGN eg. Quasars, Seyfert, BL Lac). Fainter AGN may make up the bulk of the diffuse X-ray background. The galactic objects are mainly binary stars containing either a white dwarf or neutron star. The rest are stars with enhanced solar-like activity. The number of known X-ray sources is set to increase to about 100,000 ( much the same as number of stars visible with binoculars) with the completion of the present ROSAT survey. In a twist of history, one of the early ROSAT images actuality showed the moon.