A Transition to the Soft State in GRS 1758–258

Near the end of 2001 February, the black hole candidate (BHC) GRS 1758-258 made an abrupt transition from a standard hard (low) state to a soft state. Unlike Cygnus X-1 and other BHCs, whose luminosity increases during this transition, GRS 1758-258 was dimmer after the transition. We present observations with the Proportional Counter Array on the Rossi X-Ray Timing Explorer and interpret the phenomenon in the context of a "dynamical" soft-state model. Using this model, we predicted that mass transfer from the companion had ceased and that the luminosity should decay on a timescale of a few weeks. The most recent data support this prediction, being consistent with a decay time of 28 days. The current state is consistent with the "off" state of GRS 1758-258 reported by SIGMA/Granat in 1991-1992.