MAXI/GSC detection of a weak X-ray activity from Be/X-ray binary pulsar GX 304-1

ATel #9064; M. Nakajima, K. Tanaka (Nihon U.), T. Mihara (RIKEN), S. Ueno, H. Tomida, S. Nakahira, M. Ishikawa, Y. E. Nakagawa, Y. Sugawara (JAXA), M. Sugizaki, M. Serino, W. Iwakiri, M. Shidatsu, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), N. Kawai, N. Isobe, S. Sugita, T. Yoshii, Y. Tachibana, Y. Ono, T. Fujiwara (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka (AGU), H. Tsunemi, R. Shomura (Osaka U.), H. Negoro, T. Masumitsu, T. Kawase (Nihon U.), Y. Ueda, T. Kawamuro, T. Hori, A. Tanimoto (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki (Chuo U.), M. Yamauchi, K. Furuya (Miyazaki U.), K. Yamaoka (Nagoya U.)
on 19 May 2016; 01:05 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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On 2016 May 17 (MJD 57525), the MAXI/GSC Nova-alert system (Negoro et al. 2016) detected a weak X-ray emission from the position consistent with the recurrent Be/X-ray transient pulsar, GX 304-1. Prior to this alert, the nova-alert system has issued two trigger information about the brightening of GX 304-1 on May 2 and May 11. The flux levels of the previous events were as low as &sim 8 mCrab. The day-averaged fluxes in 4-10 keV band reached 0.027 +- 0.017 photons/s/cm² (22 +- 14 mCrab) on May 17, and 0.035 +- 0.010 photons/s/cm² (28 +- 8 mCrab) on May 16. The weak X-ray activity has also been observed by Swift/BAT since May 9 (MJD 57517). In the 15-50 keV energy band, GX 304-1 has exhibited the constant flux level of 0.0046 +- 0.0009 counts/s/cm² (21 +- 4 mCrab) between May 9 (MJD 57517) and May 17 (MJD 57525). According to the orbital parameters (P_orb=132.189 d, T₀=55425.6(MJD) ; Sugizaki et al. 2015), the onset phase of the present brightening (May 17) is turned out to be 0.85. In contrast to the trend of the outburst onset delay observed for at last 5 brightening events (Atel#8592), the onset phase of the present outburst has changed to "before the periastron". Before the present event, we had expected that the present outburst start from the orbital phase of &sim 0.05, based on the steady shift of the outbursts (Atel#8592). This outburst onset transition might imply that the elongated elliptical circumstellar disc exists around the Be star. To investigate the profiles of the circumstellar disc, we strongly encourage multiwavelength observations. The latest light curve can be checked at the following page;
http://maxi.riken.jp/top/index.php?cid=1&jname=J1301-616