Title

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

Document Type

Article

Publication Date

5-21-2019

Publication Title

The Astronomer's Telegram

Issue

12788

First page number:

1

Last page number:

1

Abstract

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.

Language

English


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