Black-Hole Candidate 1E 1740.7-2942 Enters a Faint Soft State

ATel #94; D. M. Smith, Space Sciences Laboratory, U. C. Berkeley; W. A. Heindl, Center for Astrophysics and Space Science, U. C. San Diego; C. B. Markwardt and J. H. Swank, NASA Goddard Space Flight Center
on 20 May 2002; 17:02 UT
Credential Certification: David M. Smith (dsmith@ssl.berkeley.edu)

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Frequent x-ray monitoring of the black-hole candidate 1E 1740.7-2942 near the Galactic Center with the Proportional Counter Array (PCA) on RXTE shows that it has faded and softened dramatically over the last month, entering a state similar to that shown by the only other persistently active "microquasar" in the Galactic bulge, GRS 1758-258, last year (ATEL #66; also Smith et al. 2001, ApJ 554, L41).

The flux above 10 keV has dropped by an order of magnitude below anything seen since PCA monitoring started in 1996 (see the figure linked below). Recent spectral fits to a power law plus a disk blackbody show that the latter dominates the overall luminosity (when corrected for absorption). This is the first observation of a disk-dominated spectrum in this system.

There are several faint, highly absorbed IR candidates for the companion of this source (Marti et al. 2000 A&A 363, 184; Eikenberry et al. 2001 ApJ 556, 1). Deep observations of this field to look for changes in any of the sources could help identify the counterpart. In addition, x-ray and radio observations of the current state are strongly encouraged. Observers should feel free to contact the lead author for updates on the state of the source.

As in the transition seen in GRS 1758-258, the soft spectrum appears while the source is dim and fading, in contrast to the standard picture where the hard state is at low accretion rates and the soft state is seen at higher luminosity. We continue to interpret such behavior as a "dynamical" soft state, an intrinsically temporary condition caused by the shutoff of accretion material to two simultaneous flows: a standard thin disk and a hot halo/corona flow. The transition to the soft state occurs because the hot flow accretes away immediately while material in the disk can only drain away on the viscous timescale (weeks). Indeed, as predicted in ATEL #66, the soft spectrum of GRS 1758-258 showed an exponential decay with a timescale of about a month after the transition. The shutoff of the hot flow has been less complete and sudden in this case (see the enclosed figure and also Smith et al. 2001 for comparison).

RXTE/PCA soft and hard lightcurves of 1E 1740.7-2942