2. X-Ray Astronomy

The period leading up to the last IAU General Assembly was dominated in X-ray Astronomy by the results from the Einstein Observatory. This first application of a large, satellite-borne, high resolution X-ray telescope to the study of cosmic sources had led, by the end of orbital operation in April 1981, to the detection of X-ray fluxes from a wide variety of astronomical objects and the full maturing of X-ray Astronomy. During the past three years a strong scientific output has continued to flow from the analysis of the more than 5600 separate Einstein observations, many of which are now widely available via the Einstein Data Bank. In sharp contrast, the peak of activity and scientific output from Einstein will apparently be followed, in the United States, by almost a decade with no new X-ray satellite. Fortunately, successful launches have been achieved in both the Japanese and European programmes, with Tenma launched in February 1983 (to join Hakucho in orbit) followed by EXOSAT three months later. Both these new missions have attracted world-wide interest and observing time on both is at a premium. Of a range of instruments on board the Tenma satellite the most productive is proving to be the large (~800 cm2) array of gas scintillation proportional counters. These detectors, flown for the first time on Tenma (and EXOSAT), provide a factor-of-two increase in spectral resolution over the more conventional proportional counters, an advantage which is particularly valuable in resolving the key emission line and absorption features associated with the K-shell of iron, near ~7 keV. One particularly exciting result reported from this Tenma instrument was an apparent absorption feature at ~4.1 keV in X-ray burst spectra from the source 2S 1636-536 (later supported by EXOSAT observations), the feature being attributed to red-shifted iron absorption in the strong gravitational field close to the neutron star burst source! At the time of writing (December 1984) a power storage problem is limiting the operational efficiency of Tenma, but the scientific payload continues to function well.