Black-Hole Candidate GRS 1758-258 Enters an "Off" State

Twice-weekly observations of the black hole candidate GRS 1758-258 with the Proportional Counter Array (PCA) on RXTE have revealed that the source has undergone a dramatic drop in hard x-ray flux for the first time since GRANAT/Sigma saw it disappear (40-150 keV) in 1991/92 (M. Gilfanov et al. 1993, ApJ 418,844). Measurements on Feb 21 and Feb 27 in the 2.5-25 keV band showed a change from a spectrum dominated by a hard power law (index -2) to a spectrum dominated by a thermal component (blackbody kT = 0.4 keV).