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.

Light curve and spectral information for GRS 1915+105

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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