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

ATel #66; 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 13 Mar 2001; 01:18 UT
Credential Certification: David M. Smith (dsmith@ssl.berkeley.edu)

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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). The residual power-law tail (index -3) now has less than 10% of the photon flux it had on Feb 21. As is usual for black-hole soft states, the fast variability has dropped, and is now less than 8% rms.

This state is quite different than the typical "high/soft" state in that the bolometric luminosity is almost a factor of 3 lower than in the hard state (the drop is about two orders of magnitude in the Sigma band). We suggest two possible interpretations. First, that it is what we call a "dynamical" soft state (D. M. Smith, W. A. Heindl & J. H. Swank 2001, submitted to ApJ). In this model, the thin disk and halo flows are independent. The halo, which scatters disk photons into the power law, disappears as soon as accretion stops, while the thin disk persists for the viscous timescale, which appears from earlier data to be a few weeks in this source. A second interpretation is that the remaining soft flux is not from the disk but rather from an outflow; there appears to be evidence of extended emission from this source in Chandra data (W. A. Heindl & D. M. Smith, 2000, BAAS 197, 118.01). In either case, we predict that if mass transfer from the companion does not resume, the remaining soft flux should fade over the next few weeks.

Deep follow-up observations with RXTE are taking place today (Mar 12), and in these data the spectrum is much the same as it has been since Feb 27. A Chandra observation has been approved, possibly for as early as next week. The state could end at any time or could last as long as 6 months or more (GRANAT/Sigma showed the source "off" in Fall 1991 and Spring 1992, though without any other observations in between). Observations are encouraged in the radio (to search for correlated changes in the core of the associated core-and-jet radio structure), in the IR/optical (for a rare opportunity to evaluate potential companions with the accretion disk turned - or turning - off), and of course in the x-ray band.

Contact D. M. Smith at the address above for more detailed information on RXTE and Chandra observing schedules.

Postscript File: PCA Count Spectra Before & After Transition