NICER / ISS Science Nugget
for November 21, 2019

Recent flaring in GRS 1915+105 shows unusual spectral signature

One of NICER's long-term projects is to monitor the behavior of the black-hole binary system GRS 1915+105. This past week, based on a clue of possible brightening seen by JAXA's ISS payload MAXI, NICER stepped up the cadence of its observations. NICER found (see figure) the source flux to be approximately 3-5 times higher than has been seen in recent months.

However, during the evening of November 20th, 2019, NICER detected a large flare, more than 100 times brighter than the quiescent flux just last week and the brightest seen from GRS 1915 in several months. Similar to a flare that occurred earlier this fall, the spectrum of this week's flare is completely different from the non-flare emission.

We are looking at these data to better understand the physical conditions of this system and how these spectra can be so different.

Figure: The light curve of GRS 1915+105 measured by NICER over the past few days is shown in panel A. Panel B zooms in on the period of a bright flare with a duration of about 200 seconds. The green curve in panel C shows the NICER X-ray spectrum of the portion of time that GRS 1915 was not flaring, while the red curve shows the flare spectrum. Curiously, emission-line features seen in the dimmer period appear to be replaced by absorption-like features in the flare spectrum.

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