J. A. van Paradijs

Correlation of X-ray burst properties with source state in the 'atoll' source 4U/MXB 1636-53

A series of Exosat observations of the atoll source 4U/MXB 1636 - 53 shows that duration and temperature of the X-ray bursts strongly correlate with the X-ray spectral and fast variability characteristics of the persistent emission of the source. This implies that spectral shape, fast variability, and burst duration and temperature all correlate well with accretion rate M. This provides a strong argument that in the atoll sources, source-state is determined by M, just as in the Z sources. These observations also show that the persistent X-ray intensity can vary independently from the other mentioned characteristics. Therefore, intensity is probably not a good measure for the accretion rate. 16 refs.

Supersoft ROSAT sources in the galaxies

Before the X-ray surveys performed with EINSTEIN and ROSAT soft X-ray (or EUV) sources were claimed to exist (Iben 1982; Fujimoto 1982); they were looked for in the ultraviolet and indeed such sources were found in the symbiotic systems (which count as CV-like systems) and termed the hot component of symbiotics. Although the nature of these hot components has been subject to debate (in terms of either nuclear burning white dwarfs or accretion phenomena) observational facts were in many systems favouring the first scenario (Mikolajewska & Kenyon 1992). Symbiotic binaries require wide orbits in order to keep the big giant star within its Roche lobe. It is natural to look for the short-period counterpart, but it was much more difficult to detect them in the optical due to the faintness of the secondary, which is supposed to be an evolved main-sequence star or even smaller. What turns out in these systems to be predominant is the much brighter accretion disk. It was the unique chance of the satellite borne X-ray imaging instruments to discover these EUV and soft X-ray sources and with the EINSTEIN observatory the first firm candidates were found. However, complete coverage of the soft X-ray sky was needed in order to get them all and ROSAT was the instrument which mapped the whole sky.