MAXI/GSC detection of the X-ray outbursts from Be/X-ray binary pulsars, GX 304-1 and A 0535+26

ATel #8055; M. Nakajima (Nihon U.), T. Mihara (RIKEN), S. Ueno, H. Tomida, S. Nakahira, M. Kimura, 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 (Kyoto U.), Y. Tsuboi, S. Kanetou (Chuo U.), M. Yamauchi, D. Itoh (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Morii (ISM)
on 14 Sep 2015; 15:18 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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We report on X-ray outbursts from two Be/X-ray binary pulsars triggered by the MAXI/GSC nova alert system.

The outburst of GX 304-1 was triggered on 2015 September 11 (MJD 57276). According to the latest orbital ephemeris (P_orb=132.189 d, T₀=55425.6(MJD) ; Sugizaki et al. 2015), the trigger time corresponds to the orbital phase of 0.002. The MAXI/GSC data showed that the current brightening started from September 10 (MJD 57275), and the day-averaged flux in the 4-10 keV energy band reached 0.092 +- 0.007 photons/s/cm² (74 +- 6 mCrab) on September 13 (MJD 57278). The Swift/BAT monitor in the 15-50 keV energy band also detected the present brightening and the latest flux is 0.019 +- 0.001 counts/s/cm² (86 +- 5 mCrab) on September 13 (MJD 57278). Since both detectors with difference energy bands exhibited moderate flux increase at the outburst ascent phase, the current brightening is expected to be a normal outburst. Comparing with the onset of the previous outburst (Atel #7441), the present one is late for 0.05 in orbital phase. This might indicate that the configuration of the Be disc slightly changed after the last outburst. Follow-up observations in optical wavelength is strongly encouraged. The latest light curve can be checked at the following page;
http://maxi.riken.jp/top/index.php?cid=1&jname=J1301-616

A weak X-ray emission from A 0535+26 was also triggered on the same day 2015 September 11 (MJD 57276). The day-averaged flux in the 4-10 keV energy band obtained by the MAXI/GSC is 0.027 +- 0.006 photons/s/cm² (22 +- 5 mCrab) on September 11 (MJD 57276), and it decreased to 0.013 +- 0.006 photons/s/cm² (10 +- 5 mCrab) on September 12 (MJD 57277). While the Swift/BAT monitor data in 15-50 keV exhibited steady flux increase from 0.0034 +- 0.0018 counts/s/cm² (15 +- 8 mCrab) on September 8 (MJD 57273) to 0.0051 +- 0.0014 counts/s/cm² (23 +- 6 mCrab) on September 13 (MJD 57278). The orbital phase of the onset of this outburst in both energy bands is about 0.06 which is derived from following orbital parameters, P_orb=111.0 d and T₀=44943.5(MJD) (Giovannelli et al. 2013). This orbital phase matched the onset of the outburst observed at 2 orbital cycle before (Atel #7015). Although the current source flux level and the brightness increase rate do not indicate the beginning of the giant outburst, the V band monitor data (Atel #8024) expected that the source will cause a large X-ray outburst within a few days. There is an example the source exhibited a weak precursor before the giant outburst in 2009 (Atel #2277, Nakajima et al. 2014). Thus there is a possibility of a large X-ray outburst. Multi-wavelength observations are strongly encouraged to monitor this source. The latest light curve can be checked at the following page;
http://maxi.riken.jp/top/index.php?cid=1&jname=J0538+263