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MAXI/GSC detection of the outburst onset from Be/X-ray binary pulsar GX 304-1

ATel #8592; M. Nakajima (Nihon U.), T. Mihara (RIKEN), Y. Nakamura (Chuo U.), S. Ueno, H. Tomida, S. Nakahira, M. Ishikawa, Y. E. Nakagawa (JAXA), M. Sugizaki, M. Serino, M. Shidatsu, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), N. Kawai, 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.), H. Negoro, K. Tanaka, T. Masumitsu (Nihon U.), Y. Ueda, T. Kawamuro, T. Hori, A. Tanimoto (Kyoto U.), Y. Tsuboi, S. Kanetou, R. Sasaki (Chuo U.), M. Yamauchi, D. Itoh, K. Furuya (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Morii (ISM)
on 26 Jan 2016; 14:26 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 9064, 9101

On 2016 January 24 (MJD 57411), the MAXI/GSC nova-search system detected an X-ray emission from the position consistent with the recurrent Be/X-ray transient pulsar, GX 304-1. The day-averaged fluxes in 2-20 keV band are 0.083 +- 0.015 photons/s/cm2 (23 +- 4 mCrab) on January 24, and 0.070 +- 0.018 photons/s/cm2 (19 +- 5 mCrab) on January 25. Swift/BAT also have observed the present brightening from January 23. In contrast to the 2-20 keV data, the 15-50 keV flux shows gradual increase from 0.0037 +- 0.0014 counts/s/cm2 (17 +- 6 mCrab) to 0.0073 +- 0.0013 counts/s/cm2 (33 +- 6 mCrab) on January 25. The hardness ratio between Swift/BAT (15-50 keV) and MAXI/GSC (2-20 keV) is 0.11 +- 0.03 which is comparable to the value in the beginning of the previous normal outburst (Atel#8055). The trigger time corresponds to the orbital phase of 0.019 (Porb=132.189 d, T0=55425.6(MJD) ; Sugizaki et al. 2015). Thus the present event is expected to be a normal outburst. Comparing with the recent outbursts, we noticed that the onset phases of the outbursts shifted steadily through the four normal outbursts (from ~0.90 orbital phase on 2014 December to 0.02 on 2016 January). Such an effect has been observed from three Be/X-ray binaries (GS 0834-430 ; Wilson et al. 1997, EXO 2030+375 ; Wilson et al. 2002, A0535+26 ; Nakajima et al. 2014). Multiwavelength observations are strongly encouraged to monitor the systematic drift in the phases of the outburst onset. The latest light curve can be checked at the following page; http://maxi.riken.jp/top/index.php?cid=1&jname=J1301-616
The light curves of the recent six outbursts are shown after shifted at the orbital phase 0 in http://maxi.riken.jp/atel/GX304-1_NOB_shift.png