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MAXI/GSC detection of a recent low-level X-ray activity and a bright hard X-ray flare from V404 Cyg

ATel #8494; J. Sugimoto (RIKEN), H. Negoro (Nihon U.), N. Kawai (Tokyo Tech), H. Tomida, S. Nakahira, M. Ishikawa, Y. E. Nakagawa (JAXA), T. Mihara, M. Sugizaki, M. Serino, M. Shidatsu, T. Takagi, M. Matsuoka (RIKEN), M. Arimoto, T. Yoshii, Y. Tachibana, Y. Ono, T. Fujiwara (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU), H. Tsunemi, R. Imatani (Osaka U.), M. Nakajima, K. Tanaka, T. Masumitsu (Nihon U.), Y. Ueda, T. Kawamuro, T. Hori, A. Tanimoto (Kyoto U.), Y. Tsuboi, S. Kanetou, Y. Nakamura, R. Sasaki (Chuo U.), M. Yamauchi, D. Itoh, K. Furuya (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Morii (ISM) report on behalf of the MAXI team
on 1 Jan 2016; 23:36 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 8499, 8500, 8501, 8507

Since 2015 December 23, a series of hard X-rays detections of the Galactic black hole candidate V404 Cyg (GS 2023+338) were reported with Swift-BAT (Barthelmy et al 2015 GCN #18716; ATel #8455), Fermi-GBM (Jenke at al 2015, GCN #18719; ATel #8457), and INTEGRAL-IBIS/ISGRI (Malyshev et al. ATel #8458). MAXI, however, had not detected significant X-ray flares from the source by December 30 (MJD 57386). The nova-alert system triggered a weak source activity on December 29 in a 4-days timescale bin (MAXI ML [X-ray-star:418]). The 4-10 keV X-ray flux averaged over December 26 to 29 was 0.0073 +/- 0.0028 photons/cm^2/s (~ 6 mCrab), slightly higher than the X-ray flux reported by Motta et al. from INTEGRAL and Swift observations (ATel #8475).

On and After December 30, V404 Cyg increased its activity as reported by Heinz et al (ATel #8489). Multiple weak X-ray flares were detected with MAXI/GSC at 8:40, 16:23, and 22:34 on Dec 30 (UT) with 2-20 keV fluxes of 62 +/- 24, 46 +/- 23 and 42 +/- 21 mCrab, respectively. At 18:38:04 on 2015 December 31 (UT), MAXI/GSC detected a bright X-ray flare event from the source. The 2-20 keV X-ray flux averaged during the scan transit was 1055 +/- 145 mCrab.

The latest light curves and images of V404 Cyg