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Swift/XRT Detects Resuscitation of GRS 1915+105

ATel #13974; Mark Reynolds, Jon Miller, Mayura Balakrishnan (University of Michigan)
on 27 Aug 2020; 15:51 UT
Credential Certification: Mark Reynolds (markrey@umich.edu)

Subjects: Radio, Infra-Red, X-ray, Black Hole, Transient, Variables

Referred to by ATel #: 14792

GRS1915+105 was discovered in 1992 by GRANAT (Castro-Tirado et al., 1994, ApJS, 92, 469) and had been persistently X-ray bright until entering a decaying/faint period approximately 2 years ago (Negoro et al. atel #11828; Homan et al. atel #12742). Initially thought to be returning to quiescence after a ~30 year outburst, detailed study of this state suggests that GRS 1915+105 had instead entered a highly absorbed, quasi compton-thick mode of accretion (Miller et al. 2020 ApJ subm, arXiv2007.07005; Balakrishnan et al. 2020, ApJ, subm).

In a Swift/XRT windowed timing mode observation on 2020 August 26th (08:08UT), GRS 1915+105 is prominently detected. The time averaged X-ray spectrum is consistent with a hot disk blackbody (e.g., tbabs*diskbb returns Nh = (4.65 +/- 0.17)e22 cm^-2, kT = 2.15 +/- 0.09 keV, K = 7.2 +/- 1.1, chi2/nu = 498/467, errors at 90% confidence level). The observed source flux is (1.58 +0.02 -0.04)e-09 erg/s/cm^2 (0.5 - 10.0 keV). For comparison, a powerlaw model results in a significantly lower quality fit (chi2/nu = 611/467).

Prominent pulsations are visible in the lightcurve consistent with the known "heartbeat" oscillations observed from this system previously (Neilsen et al. 2011, ApJ, 737, 2; Zoghbi et al. 2016, ApJ, 833, 165). The oscillations are currently at a measured frequency of 23+/-1 mHz (90%).

We note that a rising flux is also visible in ongoing monitoring by MAXI, but that the source is not currently detected in hard X-rays by Swift/BAT.

Further observations to study the re-awakening of GRS 1915+105 are encouraged.