A NuSTAR Observation of GRS 1915+105 in its Unusual Low State

ATel #12788; J. M. Miller, M. T. Reynolds, A. Zoghbi (Univ. of Michigan); A. C. Fabian (Univ. of Cambridge), J. S. Kaastra (SRON), T. Kallman (NASA/GSFC), D. Proga (UNLV), J. Raymond (SAO)
on 21 May 2019; 14:51 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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We observed the stellar-mass black hole GRS 1915+105 with NuSTAR, in an effort to better understand the accretion flow in its current low-flux state. NuSTAR ObsID 90501331002 was obtained on 2019-05-05, starting at 07:06:09 (UT). The net exposure time was 27.5 ks.

The spectrum does not clearly require a thermal component in the 3-79 keV band, though some models are marginally improved by the inclusion of a hot disk blackbody with a very small (perhaps unphysical) emitting area.

Fits with a simple absorbed power-law model reveal what appear to be a relativistically broadened iron line, and a Compton back-scattering hump peaking in the 20-30 keV band, nominally consistent with relativistic disk reflection. However, when fit with disk reflection, strong absorption residuals remain in the Fe K band, and indicate a combination of K-a and K-b absorption lines, possibly at a range of velocity shifts. In all fits, one absorption zone requires a blue-shift of 0.05-0.1c. Even using the latest "relxill" reflection model and a set of XSTAR photoionized absorption and re-emission zones, we are unable to achieve a statistically acceptable fit (chi^2/nu > 2).
A plot of a simple power-law fit to the spectrum is shown here:
https://sites.lsa.umich.edu/jonmm/2019/05/21/nustar-on-grs-1915105/

Accurate spectral modeling must await a much longer and more detailed analysis. In rough terms, though, the power-law index appears to be about Gamma = 1.8, and the observed flux is about 1 E-9 erg/cm2/s in the 3-79 keV band.

This observation follows the absorption line-rich but Compton-thin spectrum obtained with Chandra on 2019-04-30 (ATEL #12743), and precedes the Compton-thick spectrum obtained on 2019-05-16 with Swift (ATEL #12771). GRS 1915+105 is now seen at low flux, but clearly not in a single state. The complex spectral variability that was typical when GRS 1915+105 was 1-2 orders of magnitude higher in flux, appears to still be fundamental to the source.

We thank Fiona Harrison, Karl Forster, and the NuSTAR team for granting and executing this observation.