Rise and fall of an X-ray star

The discovery (in December 1974) and the life history during 1975 of the spectacular transient x-ray source A0620-00 are described. Observations obtained from instruments aboard Ariel V and from many ground-based equipments are considered in relation to the nature of the source. The optical identification, the first ever of a transient x-ray source, was especially significant in providing a very precise position, a possible link with optical novae and the potential means of determining the distance and hence the intrinsic power of A0620-00. A radio source in the right position has also been detected. The x-ray intensity of the star during August 1975, showing a maximum on 14 August when the intensity was three times that of Scorpio X-1, is shown graphically. (U.K.)