The Black Hole GRS 1915+105 is Still Active in X-rays
ATel #15908; J. M. Miller (Univ. of Michigan), J. Homan (Eureka Scientific)
on 17 Feb 2023; 21:34 UT
Credential Certification: Jon Miller (jonmm@umich.edu)
Subjects: X-ray, Binary, Black Hole
We report on an observation of the stellar-mass black hole GRS 1915+105, obtained with the Neil Gehrels Swift Observatory on 17 February 2023. This observation was requested as the source flux is currently steadily consistent with zero in the Swift/BAT, and is not clearly detected with MAXI. A net exposure time of 0.86 ks was obtained.
We fit the spectrum in the 1-10 keV band after binning to require S/N = 3, minimizing a Cash statistic. A simple absorbed power-law model, tbabs*pow, gives a power-law index of Gamma = 1.1 +/- 0.6, a column density of N_H = 22 +/- 7 E+22 cm^-2, and an unabsorbed flux of 9.7 +/- 1.1 E-11 erg/cm2/s. For comparison, a fit with the same model (power-law index fixed to 1.1) to the spectrum of the previous observation (obtained on 15 October 2022) resulted in a column density of N_H = 39 +/- 5 E+22 cm^-2 and an unabsorbed flux of 4.4 +/- 0.7 E-11 erg/cm2/s (a factor of ~2 lower than in the Feb 17 observation).
This simple power-law model allows for unphysically flat power-law indices, and an implausible ISM column density. We next adopted a model with a fixed line-of-sight absorption (N_H = 4 E+22 cm^-2), internal absorption of a cut-off power-law (Gamma = 2 and E_cut = 100 keV fixed), and a pexmon reflection component with a reflection fraction of unity and other parameters tied to the power-law: tbabs*(tbabs*cutoffpl + pexmon). This model makes contact with the models used by Balakrishnan et al. (2021) to trace the evolution of GRS 1915+105 into its current obscured state (also see Miller et al. 2020). For the most recent observation we measure an internal column density of N_H = 3.0 +/- 0.5 E+23 cm^-2 and an unabsorbed flux of F = 1.4 +/- 0.3 E-10 erg/cm^2/s. The implied column is on the low end of the range found in prior fits to this state.
We encourage monitoring of GRS 1915+105 in X-rays using the XRT, and in radio bands.
We thank Brad Cenko and the Swift team for executing this observation as soon as GRS 1915+105 was visible with the XRT.
References
Balakrishnan, M., et al., 2021, ApJ, 909, 41
Miller, J. M., et al., 2020, ApJ, 904, 30
