MAXI/GSC observation of X-ray brightening from Be/X-ray binary pulsar GRO J1008-57

ATel #11244; M. Nakajima (Nihon U.), T. Mihara (RIKEN), M. Serino (AGU), Y. Tachibana (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), M. Sugizaki, S. Nakahira, W. Iwakiri, M. Shidatsu, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), N. Kawai, S. Sugita, T. Yoshii, S. Harita, Y. Muraki, K. Morita (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka, T. Hashimoto (AGU), H. Tsunemi, T. Yoneyama (Osaka U.), H. Negoro, T. Kawase, A. Sakamaki (Nihon U.), Y. Ueda, T. Hori, A. Tanimoto, S. Oda, T. Morita, S. Yamada (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki, H. Kawai, T. Sato (Chuo U.), M. Yamauchi, C. Hanyu, K, Hidaka (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.)
on 30 Jan 2018; 12:14 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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MAXI/GSC nova-alert system (Negoro et al. 2016) detected X-ray brightening from the Be/X-ray binary pulsar GRO J1008-57 on 2018 January 27 (MJD 58145). According to the MAXI/GSC monitor data, the current brightening has started from January 23 (MJD 58141). The onset of this event corresponds to the orbital phase of 0.90, which is derived from the the orbital parameters, T₀=54416.65 (MJD) and P_orb=249.48 d (Kühnel et al. 2013). The 4-10 keV energy flux increased from 0.019 +- 0.06 photons/s/cm² (17 +- 5 mCrab) on January 23 (MJD 58141) to 0.045 +- 0.008 photons/s/cm² (39 +- 7 mCrab, ) on January 29 (MJD 58147). The average flux increase rate in recent days is ~0.004 photons/s/cm²/day. Above results indicate that the present brightening is normal (type-I) outburst.

In 2009-2015 season, the MAXI/GSC observations revealed that the onset phase of the normal outbursts distributed in a narrow range (0.92-0.93) in orbital phase. However, the last 4 normal outbursts (Atel#8547,#9512,#10418,#10466) started from the orbital phase of 0.91 (The light curves of the recent 4 outbursts are shown after shifted at the orbital phase 0 in http://134.160.243.88/news/en/MAXI_News_1_files/groj1008_20180130_gscmed.gif ). We have confirmed that same trend exists in the Swift/BAT monitor data. This outburst onset shift might imply that the geometrical parameters of the circumstellar disc around the Be star have changed. To investigate the profiles of the Be disc, we encourage multiwavelength observations.

The X-ray light-curve can be followed at the following pages:
MAXI (http://maxi.riken.jp/top/index.php?cid=1&jname=J1009-582),
Swift/BAT (http://swift.gsfc.nasa.gov/results/transients/GROJ1008-57/),
Fermi/GBM (https://gammaray.nsstc.nasa.gov/gbm/science/pulsars/lightcurves/groj1008.html), and
the BeXRB monitor page (http://integral.esac.esa.int/bexrbmonitor/Plots/sim_plot_GROJ1008-57.html).