Evidence of an anomalous state in the black hole candidate 4U 1630-47

We have reanalyzed two EXOSAT medium-energy observations during part of the 1984 X-ray outburst of the black hole candidate transient system 4U 1630-47. One observation (May 10) shows fast-timing properties (on timescales shorter than similar to 250 s) typical of those of other black hole candidates in their high states. The power spectrum shows a power law with index similar to 0.7 and a fractional rms amplitude (0.001-1 Hz) of similar to 2%. Another observation, obtained 1 month earlier (April 11), shows different fast-timing behavior. The power spectrum of this observation can be described by a power law with index similar to 1.5 and a fractional rms amplitude (0.001-1 Hz) of similar to 7%. This is inconsistent with the behavior during low, high, or very high states known to occur in most of the black hole candidates. Instead, the timing properties resemble those recently reported from the Advanced Satellite for Cosmology and Astrophysics observations of the superluminal black hole candidates GRS 1915+105 and GRO J1655-40 (X-ray Nova Sco 1994). This timing behavior may represent a new state of black hole candidates. In addition, the X-ray spectra of the three sources show similar properties. Since GRS 1915+105 and GRO J1655-40 exhibit superluminal jets, we propose that 4U 1630-47 is also a relativistic jet source.