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GRS 1758-258: Into the thermal dominant state with Swift XRT

ATel #9890; M. Hirsch (ECAP-FAU), K. Pottschmidt (NASA-GSFC/UMBC), F. Krauss (UvA), W. Eikmann, I. Kreykenbohm, J. Wilms, M. Kuehnel (all ECAP-FAU), B. H. G. Rodrigues (Itajuba), R. Soria (ICRAR-Curtin/Sydney), V. Grinberg (ESA-ESTEC), D. M. Smith (UCSC), M. Cadolle Bel (MPCDF), J. A. Tomsick (UCB), A. Bodaghee (GSCU), E. Kuulkers (ESA-ESTEC) on behalf of the INTEGRAL Galactic bulge monitoring team, E. Kalemci (Sabanci), J. M. Miller (Michigan)
on 21 Dec 2016; 17:02 UT
Credential Certification: Katja Pottschmidt (katja@milkyway.gsfc.nasa.gov)

Subjects: X-ray, Black Hole

A decline of the hard flux of the Galactic black hole candidate GRS 1758-258 was observed with Swift BAT and INTEGRAL ISGRI starting in 2016 mid-August, with the source having become undetectable above 20 keV around 2016 September 30 (ATEL #9625).

The source remained undetectable in Swift BAT until around 2016 mid-November. Since then the hard flux seems to be in the process of recovering. Since the source is currently in the solar exclusion zone, however, the Swift BAT flux uncertainties are large and no pointed observations are possible.

We report on Swift XRT observations in the soft energy band that were obtained during the decline (2.7 ks on 2016-09-24) and non-detection (2.4 ks on 2016-10-23, 2.7 ks on 2016-10-28, and 2.5 ks on 2016-11-01) of the hard flux, and compare them to a typical hard state observation (3.3 ks on 2009-09-10).

All spectra can be described with a disk black body plus power law model modified by interstellar absorption as well as intrinsic, partial covering absorption, a typical model for the source. Comparing to Soria et al. (2011, MNRAS 415, 410) we identify the observation from 2016 September as an intermediate state and the three 2016 October/November observations as soft states. Absorbed black body or power law models do not yield good descriptions, with the exception of the latter in the hard state case. Comptonization models are not very successful, either. Due to parameter degeneracies of the bestfit model we illustrate the main differences between the observations using model independent, absorbed fluxes, for now:

In the intermediate state, the unabsorbed 0.3-12 keV flux, 7.4 x 10^-10 erg/s/cm^2, is moderately higher than in the hard state, 6.1 x 10^-10 erg/s/cm^2, mainly due to an increase below 2.5 keV: The 0.3-2.5 keV unabsorbed flux changed from 6.5 x 10^-11 erg/s/cm^2 (hard state) to 1.7 x 10^-10 erg/s/cm^2 (intermediate), while the 2.5-12 keV unabsorbed flux remained comparatively similar with 5.5 x 10^-10 erg/s/cm^2 (hard state) and 5.7 x 10^-10 erg/s/cm^2 (intermediate state).

In the soft state, the unabsorbed 0.3-12 keV flux is considerably lower, 2.5-3.0 10^-10 erg/s/cm^2, in this case a decline above 2.5 keV is primarily responsible: The 2.5-12 keV unabsorbed flux changed from 5.7 x 10^-10 erg/s/cm^2 (intermediate state) to (1.3-1.6) x 10^-10 erg/s/cm^2 (soft state), while the 0.3-2.5 keV unabsorbed flux remained comparatively similar with 1.7 x 10^-10 erg/s/cm^2 (intermediate state) and (1.2-1.4) x 10^-10 erg/s/cm^2 (soft state).

Together with the 2001 soft state (Smith et al. 2001, ApJ 554, L41; Soria et al. 2011, MNRAS 415, 410) this is only the second time that GRS 1758-258 was observed to make a full transition to the thermal dominant state, in the sense that the >20 keV emission became undetectable for weeks. (Less severe softening has been observed at other occasions, though, see, e.g., Pottschmidt et al., 2006, A&A, 452, 285.)

We encourage observations to determine the degree of recovery toward the hard state as soon as the source becomes visible again to X-ray missions in 2017.