MAXI/GSC detection of a new outburst from the Galactic black hole candidate GS 2023+338 (V* V404 Cyg)

ATel #7646; H. Negoro, T. Matsumitsu (Nihon U.), T. Mihara, M. Serino, M. Matsuoka (RIKEN), S. Nakahira, S. Ueno, H. Tomida, M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA), M. Sugizaki, M. Shidatsu, J. Sugimoto, T. Takagi (RIKEN), N. Kawai, T. Yoshii, Y. Tachibana (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU), H. Tsunemi, R. Imatani (Osaka U.), M. Nakajima, K. Tanaka (Nihon U.), Y. Ueda, T. Kawamuro, T. Hori (Kyoto U.), Y. Tsuboi, S. Kanetou (Chuo U.), M. Yamauchi, D. Itoh (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Morii (ISM)
on 17 Jun 2015; 03:20 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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On 2015 June 16 (MJD 57189), the MAXI/GSC nova alert system detected burst-like activities from the position consistent with the Galactic black hole candidate GS 2023+338 (V404 Cyg, Makino et al. IAUC. #4786). This renewed activity from the source was first detected by Swift/BAT at 18:31 on June 15 (Barthelmy et al. GCN #17929). Currently, MAXI observed the region with the degraded and uncalibrated counter, GSC3. The GSC3 clearly detected bright X-ray flare(s) from the source in the scan transits at 18:48 and 20:21 on June 16. The 4-10 keV fluxes were roughly 500-600 mCrab. In the following scan transits at 21:53 and 23:26 on June 6 and 00:59 on June 7, however, the flux decreased again to ~100 mCrab or less. Note that the detection limit in a single scan transit with the GSC3 is roughly 100 mCrab.

Such sporadic flare activities of this source were often observed in the previous outburst in 1989 (Terada et al. 1994, PASJ 46, 477; Oosterbroek et al. 1997, A&A 321, 776). We strongly encourage multi-wavelength followup observations to observe this unique black hole candidate.