MAXI/GSC observes GRS 1915+105 in the X-ray faintest state in the last 22 years

ATel #11828; H. Negoro (Nihon U.), Y. Tachibana, N. Kawai (Tokyo Tech), K. Yamaoka (Nagoya U.), Y. Ueda (Kyoto U.), M. Nakajima, A. Sakamaki, W. Maruyama (Nihon U.), T. Mihara, S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), M. Sugizaki, K. Morita (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), A. Tanimoto, T. Morita, S. Yamada (Kyoto U.), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo U.), H. Tsunemi, T. Yoneyama (Osaka U.), M. Yamauchi, K. Hidaka, S. Iwahori (Miyazaki U.), T. Kawamuro (NAOJ), M. Shidatsu (Ehime U.)
on 9 Jul 2018; 02:24 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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MAXI/GSC observations of the microquasar GRS 1915+105 show that the X-ray flux is nearly exponentially decreasing in the last >100 days. Currently, the source is in the faintest state in the soft (< 10 keV) X-ray band in the last 22 years of continuous monitoring with RXTE/ASM and MAXI/GSC.

The GSC spectrum obtained from 20:14 on 2018 July 7 (MJD 58306) to 13:17 July 8 is well described by an absorbed power-law model with a photon index of 2.67 +/- 0.40 and a hydrogen column density nH of (5.4 +/- 2.0)e22 atoms/cm2, or by an absorbed disk blackbody (diskbb) model with an inner disk temperature of 2.2 +/- 0.4 keV and nH of (1.7 +/- 1.4)e22 atoms/cm2. The absorbed 2-10 keV X-ray flux in the power-law fit is 2.4e-9 erg/s/cm2 (unabsorbed power-law model, 4.3e-9 erg/s/cm2). The hard X-ray flux observed with Swift/BAT is also declining, but it is not the historically faintest (https://swift.gsfc.nasa.gov/results/transients/GRS1915p105/). We note that the hard X-ray flux obtained with Swift/BAT and the 15-50 keV/ 2-10 keV ratios obtained with Swift/BAT and MAXI/GSC show an unusual negative correlation, and the ratio is the highest ever.

The present source activity may be classified of class "chi" according to the classification of variability by Belloni et al. (2000, A&A 355, 271), or is in a soft state shown by Ueda et al. (2009, ApJ 695, 888). Continuous short-term variability study is required to confirm it. Peculiar properties of this source among black hole binaries are often attributed to its high mass accretion rate. The long-term exponential decay in X-ray flux just similar to other X-ray novae might suggest the end of the outburst of this long-lived X-ray nova. We note, however, that such declines were observed a few times previously and later the flux increased again.
Thus, we encourage multi-wavelength observations to confirm the current status of the source and to understand the anomalous nature of the microquasar GRS 1915+105.

MAXI/GSC data of GRS 1915+105