MAXI/GSC detection of X-ray rebrightening of GRS 1915+105

ATel #13652; M. Aoki, H. Negoro, K. Kobayashi, M. Nakajima, R. Takagi, K, Asakura, K, Seino, S. Mokumoto (Nihon U.), T. Mihara, C. Guo, Y. Zhou, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, R. Adachi, M. Niwano (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka (JAXA), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), and M. Sugizaki (NAOC)
on 22 Apr 2020; 04:04 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The MAXI/GSC nova alert system triggered on X-ray rebrightening of the microquasar GRS 1915+105 at 00:54 UT on 2020 April 20. GSC data show that the X-ray flux has been gradually increasing since around April 16, MJD 58955. The 2-4 keV, 4-10 keV, and 10-20 keV count fluxes on April 21 are 0.037 +/- 0.007 ph/cm^2/s (~ 22 mCrab), 0.082 +/- 0.007 ph/cm^2/s (~ 71 mCrab), and 0.023 +/- 0.007 ph/cm^2/s (~64 mCrab), respectively.

A longterm trend of decrease in X-ray flux of GRS 1915+105 has been observed with the MAXI/GSC since around February 2018, MJD 58200 (ATel #11828, #12761, also see ATel #12742, #12743, #12755, #12765, #12769, #12771, #12772, #12773, #12788). After the relatively stable flux period on MJD 58661-58782 at 20-30 mCrab, very bright radio and intense X-ray flares were observed on 2019 November 19-21, MJD 58806-58808, with RANTAN-600 (ATel #13304), NICER (ATel #13308) and Konus-Wind (ATel #13316). Simultaneously, MAXI/GSC data showed further X-ray flux decrease to 10-20 mCrab or less at 2-20 keV. From 2020 January 27, MJD 58875, the X-ray flux was sometimes below the detection limit of the MAXI/GSC, 5-6 mCrab a week though the brightest radio flare (ATel #13442) and an intense X-ray flare (ATel #13478) were also detected on February 2 and 13, respectively,

The latest 4-10 keV flux about 100 mCrab is comparable to the flux before the rapid decrease in X-rays starting from 2019 April (MJD ~58600) (e.g., ATel #12742, #12743, #12755, #12848) with many intense X-ray flares (e.g., ATel #12761, #12770, #12787, #12793, #12805, #12818, also see ATel #12806, #12839, #12855 for NIR and radio activities). This gradual increase in X-ray flux may indicate the onset of renewed activity of this source. Followup observations are encouraged.

MAX GSC data for GRS 1915+105