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GRS 1758-258 not detected at hard X-rays

ATel #9625; K. Pottschmidt (NASA-GSFC/UMBC), W. Eikmann, I. Kreykenbohm, J. Wilms, M. Hirsch, M. Kuehnel (all ECAP-FAU), B. H.G. Rodrigues (Itajuba), F. Krauss (UvA), R. Soria (ICRAR-Curtin/Sydney), V. Grinberg (MIT), D. M. Smith (UCSC), M. Cadolle Bel (MPCDF), J. A. Tomsick (UCB), A. Bodaghee (GSCU), E. Kuulkers (ESA-ESAC) on behalf of the INTEGRAL Galactic bulge monitoring team, E. Kalemci (Sabanci), J. M. Miller (Michigan)
on 14 Oct 2016; 18:01 UT
Credential Certification: Katja Pottschmidt (katja@milkyway.gsfc.nasa.gov)

Subjects: X-ray, Black Hole

Referred to by ATel #: 9890

The Galactic black hole candidate GRS 1758-258 is usually found in the hard state but has been known to enter soft states in which the spectrum changes from a power-law to a thermal disk without substantial changes in bolometric luminosity (Soria et al., 2011, MNRAS 415, 410). The most extended soft state to date was observed in 2000-2002 (Smith et al., 2001, ApJ 554, L41). The soft states are associated with a strong drop in flux above 3 keV (Pottschmidt et al., 2006, A&A 452, 285; Soria et al., 2011, MNRAS 415, 410). Recent observations of the source show the strongest drop in flux since the 2000-2002 episode:

The Swift BAT 15-50 keV light curve of GRS 1758-258 shows a rapid decline in flux over the past ~50 days and the source has not been detected during the past ~15 days ( http://swift.gsfc.nasa.gov/results/transients/GRS1758-258/ ).

The source has been extensively covered by the INTEGRAL Galactic bulge monitoring-, Galactic Center monitoring-, and our GRS 1758-258 TOO observation program from 2016 August 17 onward. INTEGRAL ISGRI confirms the rapid flux decline over this time range. Selected values in the 20-40 keV energy band are 69+/-3 mCrab (8.7+/-0.3 cps) on 8/17, 24+/-2 mCrab (3.0+/-0.2 cps) on 9/2, and 15+/-1 mCrab (1.9+/-0.2 cps) on 9/27. Selected values in the 40-80 keV energy band are 74+/-4 mCrab (4.8+/-0.3 cps) on 8/17 31+/-2 mCrab (2.0+/-0.2 cps) on 9/2, and 13+/-2 mCrab (0.9+/-0.1 cps) on 9/27.

GRS 1758-258 has not been detected by ISGRI since 2016 September 30. During the most recent INTEGRAL Galactic Center observation on 2016 October 11, with an effective exposure of 4.4 ks, upper limits of 0.4 cps and 0.5 cps were obtained in the 20-40 keV and 40-80 keV energy bands, respectively.

Additional observations are encouraged.