A Transition to the Soft State in GRS 1758−258

Near the end of 2001 February, the black-hole candidate (BHC) GRS 1758-258made an abrupt transition from a standard hard (low) state to a soft state.Unlike Cyg X-1 and other BHCs, whose luminosity increases during thistransition, GRS 1758-258 was dimmer after the transition. We presentobservations with the Proportional Counter Array on the Rossi X-ray TimingExplorer and interpret the phenomenon in the context of a ``dynamical'' softstate model. Using this model we predicted that mass transfer from thecompanion had ceased, and that the luminosity should decay on a timescale of afew weeks. The most recent data support this prediction, being consistent witha decay time of 34 dy. The current state is consistent with the ``off'' stateof GRS 1758-258 reported by GRANAT/Sigma in 1991-1992.Comment: 13 pages, 5 figures, accepted for publication in The Astrophysical Journal Letters Accepted version has only minor changes, plus extra data showing more of the deca