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A Bright Flare from GRS 1915+105 with NICER

ATel #12793; J. Neilsen (Villanova), J. Homan (Eureka Scientific and SRON), K. Gendreau, Z. Arzoumanian (NASA/GSFC), J. F. Steiner (SAO), D. Altamirano (University of Southampton), S. Eikenberry (U. Florida), R. Remillard (MIT), W. Iwakiri (Chuo U.), A. C. Fabian (Univ. of Cambridge)
on 22 May 2019; 15:00 UT
Credential Certification: Joey Neilsen (jneilsen@villanova.edu)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 12806, 12839, 12848, 13308, 13478, 13652

The black hole X-ray binary GRS 1915+105 has been in outburst for 27 years (Castro-Tirado et al. 1992), but has dimmed dramatically over the last year and may be approaching quiescence (ATel #11828, #12472, #12473, #12755). As such, there has been a renewed focus on multiwavelength observations of the source, revealing radio and infrared flaring (ATel #12769, #12773), an X-ray halo (ATel #12770), and strong emission/absorption lines (ATel #12788).

As part of a NICER Cycle 1 GO program (PI: Neilsen) we have been monitoring the source in its decay (see also Neilsen et al. 2018, ApJ, 860, L19). NICER observations on May 18-19 showed the source brightening slowly by a factor of ~3, from roughly 5 cts/s in the 0.25-15 keV band (approximately 0.5 mCrab, the lowest count rate observed with NICER for the source) to 15 cts/s, and then begin to fade. However, an observation beginning late on May 20 found the source at ~50 cts/s just before the start of a bright flare. The flare lasted about 200 seconds, with a bright rapid peak lasting approximately 37 seconds and reaching about 3400 cts/s in 1-second bins.

Analysis of the X-ray spectrum reveals significant spectral evolution of GRS 1915+105 during this period. The spectrum before and after the flare has a hard X-ray continuum with a very strong, skewed iron line typical of relativistic reflection, with a narrow component at ~6.4 keV. The average spectrum of the flare itself shows a deep iron absorption line at ~6.65 keV. Using the multiplicative absorption model gabs, we find an optical depth in this feature tau=0.36. Given the unusual behavior of the source, as well as systematic uncertainties in the gain above 6 keV, it is not clear how the absorption connects to known disk winds and ionized ejecta in GRS 1915. The line energy matches the He-alpha line of Fe XXV, an ion that is not usually observed without accompanying Fe XXVI absorption (though this depends somewhat on the local ionizing radiation field). We therefore also explored models featuring a second absorption line located at 7 keV. These models require the bright skewed emission line to be present during the flare as well as before and after; the absorption lines are superimposed on top of the emission line. The inferred optical depth of the 7 keV line is tau~0.22. More detailed analysis will be required to determine the best model of the continuum and lines, and the implications of these features for accretion/ejection processes in GRS 1915+105.

Images of the flare and spectra described here may be found at the link below.

Additional NICER observations are planned, and we encourage further monitoring at other wavelengths.

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.

Images of the flare lightcurve and spectra from NICER.