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NICER detection of a strong X-ray flare from GRS 1915+105

ATel #13308; J. Homan (Eureka Scientific & SRON), J, Neilsen (Villanova), K. Gendreau, Z. Arzoumanian (NASA/GSFC), R. Remillard (MIT), J. Steiner (SAO), D. Altamirano (University of Southampton), M. T. Wolff (NRL)
on 22 Nov 2019; 18:35 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 13316

Earlier this year, the black hole X-ray binary GRS 1915+105 entered an unusual low-flux state (ATel #12742). The source has likely become highly obscured (ATel #12771, #12848), but occasional bright X-ray flares are observed (ATel #12793, #12805). NICER has been observing GRS 1915+105 on a regular basis in its current state (~5 ks per week on average in the last three months).

Prompted by indications of a possible brightening in MAXI and Swift/BAT data, we observed GRS 1915+105 with NICER for a total of ~9.6 ks between Nov 19 19:54 UT and Nov 21 02:54 UT. This observation revealed a strong X-ray flare that started on Nov 20 at ~04:47 UT and lasted for ~400 s. The flare reached a peak 0.5-12 keV count rate of ~3750 count/s (at a time resolution of 1 s), making it the brightest X-ray flare detected with NICER since the source entered its current low flux state. For comparison, the persistent count rate just before and after the flare was variable but much lower, between 30 and 100 counts/s. A figure showing a light curve of the NICER observation is linked below.

The spectral evolution of the flare is comparable to a similar but fainter flare reported in 2019 May (ATel #12793). The off-flare spectrum is complex, with a flat continuum, a strong Fe edge above 7 keV and several prominent emission lines (near 6.4, 6.7, and 7 keV, typical of cold Fe, Fe XXV, and Fe XXVI, respectively). In contrast, the flare emission is much softer and is marked by strong absorption lines. The two most prominent (at ~6.6 keV and 7 keV) are likely He- and H-like iron, but we also detect features attributable to Si, Ar, and Ca. The spectral continuum at the peak of the flare can be fit well with an absorbed power law (nH=7.4e22, gamma=1.4), yielding an unabsorbed 0.5-12 keV flux of ~8.9e-8 erg/cm^2/s. For a distance of 8.6 kpc (Reid et al. 2014, ApJ, 796, 8), this gives a 0.5-12 keV luminosity of ~7.9e38 erg/s.

Trushkin et al. (ATel #13304) reported the detection of a strong, optically thin radio flare that started between Nov 20 12:31 UT and Nov 21 12:27 UT. It is possible that the X-ray flare observed with NICER early on Nov 20 was a precursor to this radio flare.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.

NICER light curve of strong flare from GRS 1915+105