Evidence for an Anomalous State in the Black Hole Candidate 4U 1630−47

We have re-analyzed two EXOSAT Medium Energy observations during part of the1984 X-ray outburst of the black-hole candidate transient system 4U 1630-47.One observation (May 10) shows fast timing properties (on timescales shorterthan ~250 s) typical of those of other black hole candidates in their highstates. The power-spectrum shows a power law with index 0.7 and fractional rmsamplitude (0.001-1 Hz) of 2%. Another observation, obtained one month earlier(April 11), shows different fast timing behavior. The power spectrum of thisobservation can be described by a power law with index 1.5 and fractional rmsamplitude (0.001-1 Hz) of 7%. This is inconsistent with the behavior duringlow, high or very high states known to occur in most of the black holecandidates. Instead, the timing properties resemble those recently reportedfrom ASCA observations of the superluminal black-hole candidates GRS 1915+105and GRO J1655-40 (X-ray Nova Sco 1994). This timing behavior may represent anew state of black hole candidates. In addition, the X-ray spectra of the threesources show similar properties. Since GRS 1915+105 and GRO J1655-40 exhibitsuperluminal jets, we propose that 4U 1630-47 is also a relativistic jetsource.