Periodic Outbursts from the Ultrasoft X-Ray Transient 4U 1630−47

Using ROSAT PSPC and Einstein solid state spectrometer and monitor proportional counter archival data, we have found two previously unreported outbursts from the ultrasoft X-ray transient 4U 1630-47 that occurred in 1979 March and 1992 September. The times of these outbursts are consistent with a previously reported, approximately 600 day outburst recurrence period, as is the subsequent nondetection of the source by ROSAT in 1993 March and August. Future outbursts are predicted to occur in 1995 December and 1997 August. The object 4U 1630-47 is a member of the class of transient X-ray binary systems that are thought to contain black holes. The 601.7 ± 3.0 day recurrence period of 4U 1630-47 is by far the shortest known for this class. The origin and possible triggers for the outbursts are discussed.