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A sudden dimming of GRS 1915+105 in X-rays

ATel #12742; J. Homan (Eureka Scientific and SRON), J. Neilsen (Villanova), J, Steiner (SAO), R. Remillard (MIT), D. Altamirano (University of Southampton), K. Gendreau, Z. Arzoumanian (NASA/GSFC)
on 10 May 2019; 14:10 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

Subjects: Black Hole, Transient

Referred to by ATel #: 12755, 12761, 12765, 12769, 12770, 12772, 12773, 12806, 12839, 12848, 13308, 13652, 13974, 14792

The black hole X-ray binary GRS 1915+105 has been in outburst since its discovery in 1992 by Castro-Tirado et al. (1992, IAUC 5590 ). In July of 2018 Negoro et al. (ATel #11828) reported that GRS 1915+105 had entered its faintest state in soft X-rays (<10 keV) in 22 years. Continued monitoring with MAXI/GSC showed that, until recently, the source had settled at a fairly stable level of ~0.08 Crab. Swift/BAT monitoring shows that at higher energies (15-50 keV) the source had entered an extended low-luminosity phase as well, and, until recently, had been at a level of ~0.15 Crab. Note that the latter does not present the lowest level seen in Swift/BAT data, which is ~0.01 Crab. MAXI/GSC and Swift/BAT data from the past few weeks suggest a dimming of GRS 1915+105 with respect to its plateau level of the past year.

We have been observing GRS 1915+105 with NICER on a weekly basis for over a year (see, e.g., Neilsen et al. 2018, ApJ, 860, L19). Observations made since early April 2019 suggest a rather sudden evolution in the source. The time averaged 0.5-10 keV count rates have dropped from 175 cts/s on April 10 to 55 cts/s on May 5. This was accompanied by spectral hardening. We fit NICER spectra from April 10, April 23, and May 5 with a model consisting of an absorbed power-law with a high-energy cut-off and an absorption line around 6.6 keV. When tying the Nh between the three spectra (~5.8e22 cm^-2) the power-law index was found to decrease from 1.83(1) to 1.45(1), while the unabsorbed 0.5-10 keV flux decreased from ~3.8e-9 erg/cm^2/s to ~1.2e-9 erg/cm^2/s. Judging from the X-ray light curves the source has also entered a different variability state (Belloni et al. 2000, MNRAS, 355, 271), although the short segments prevent us from making a firm classification. The ~1-2 Hz type-C QPOs that were present before the recent dimming appear to be absent at the lowest observed count rate levels.

NICER, RXTE/ASM, MAXI/GSC, and Swift/BAT light curves of GRS 1915+105 are plotted in the figure linked below.

Swift/XRT has observed the source on a daily basis since May 7. These observations reveal an overall, but not monotonic, decline in the 0.3-10 keV count rate by ~10%.

It is not clear if the recent, sudden dimming of GRS 1915+105 in X-rays represents a true decline toward quiescence, or if this behavior is temporary. We strongly encourage multi-wavelength observations to continue following the evolution GRS 1915+105.

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.

Light curves of GRS 1915+105