M ar 2 00 1 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 Cyg 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 34 dy. The current state is consistent with the “off” state of GRS 1758–258 reported by GRANAT/Sigma in 1991-1992. Subject headings: accretion, accretion disks — black hole physics — x-rays:stars — stars,individual:(GRS 1758-258)