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

ATel #4420; M. Nakajima (Nihon U.) T. Mihara, M. Sugizaki (RIKEN), S. Ueno, H. Tomida, S. Nakahira, M. Ishikawa (JAXA), M. Serino, T. Yamamoto, J. Sugimoto, M. Matsuoka (RIKEN), N. Kawai, M. Morii, R. Usui, K. Ishikawa (Tokyo Tech), A. Yoshida, T. Sakamoto (AGU), H. Tsunemi, M. Kimura (Osaka U.), H. Negoro, M. Asada, H. Sakakibara, N. Serita (Nihon U.), Y. Ueda, K. Hiroi, M. Shidatsu, R. Sato (Kyoto U.), Y. Tsuboi, M. Higa (Chuo U.) M. Yamauchi, Y. Nishimura, T. Hanayama, K. Yoshidome (Miyazaki U.), K. Yamaoka (Waseda U.), report on behalf of the MAXI team:
on 25 Sep 2012; 06:38 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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

Referred to by ATel #: 4812

On September 21, 2012, MAXI Nova-search system detected an X-ray flux increase from a Be/X-ray binary pulsar, GX 304-1. The X-ray light curve obtained by the MAXI/GSC all-sky survey reveals that the brightening started on September 17 and reached 40 +- 5 mCrab (4-10 keV) on September 23.
According to the previous activities of this source ( Atel #1613, #2297, #2538, #2779, #3075, #3087, #3309, #3624, #3856 ), the current outburst corresponds to the 12th normal outburst following the previous 11 outbursts that repeated by a 132.5-day orbital period (Priedhorsky et al. 1983; Yamamoto et al. 2011) since 2008. Although these previous outbursts recycled by the 132.5-day period, the current outburst began 15 days earlier than that expected from the recurrent cycle. This strange behavior might have started from the previous outburst, which exhibited a double-peaked light-curve profile. Such a double-peaked outburst has ever been observed in A 0535+26, which developed into a giant outburst in the next cycle (e.g. Moritani et al. 2011). Therefore, we consider that GX 304-1 will possibly go into a giant outburst this time. We encourage optical observations to monitor the V/R profiles of emission lines.
The folded MAXI/GSC X-ray light curves by the orbital period is displayed at http://maxi.riken.jp/news/en/ The light curve of the current start of an outburst traces that of the precursor of A0535+26 in a giant outburst very well.
The latest light curve and the image are available at the following URL: http://maxi.riken.jp/top/index.php?cid=1&jname=J1301-616