MAXI/GSC detection of an onset of apastron outburst from Be/X-ray binary pulsar GRO J1008-57

ATel #10902; M. Nakajima (Nihon U.), R. Sasaki (Chuo U.), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), T. Mihara, M. Sugizaki, S. Nakahira, W. Iwakiri, M. Shidatsu, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), N. Kawai, S. Sugita, T. Yoshii, Y. Tachibana, S. Harita, Y. Muraki, K. Morita (Tokyo Tech), A. Yoshida, T. Sakamoto, M. Serino, 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, H. Kawai (Chuo U.), M. Yamauchi, C. Hanyu, K, Hidaka (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.)
on 28 Oct 2017; 04:12 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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The weak X-ray brightening from Be/X-ray binary pulsar GRO J1008-57 was triggered by the MAXI/GSC nova-alert system (Negoro et al. 2016) on 2017 October 25 (MJD 58051). Following the previous anomalous activities in this orbital cycle (Atel#10529,#10533,#10561,#10617,#10626), the source again exhibited the X-ray flux enhancement at the orbital phase of 0.54 which is derived with the orbital parameters, T₀=54416.65 (MJD) and P_orb=249.48 d (Kühnel et al. 2013). This is the third X-ray brightening since the last periastron passages in 2017 May (Atel#10418,#10466). The day-averaged 4-10 keV flux reached to 0.051+/-0.028 photons/s/cm² (44+/-24 mCrab) on 2017 October 23 (MJD 58049). Since the vicinity of the source region went into the edge of the GSC FOV after the trigger (see http://134.160.243.88/pubdata/v4l/skyimg/skyimg_rgb/gscsum_r2pcl_mjd58048_colphi38.0.png ), we can not monitor its flux variation precisely in the present few days. The MAXI/GSC observation of GRO J1008-57 will be resumed from the end of October. The other X-ray observatories, Swift/BAT and Fermi/GBM, also have detected the brightening from this source. This source had exhibited the apastron X-ray outburst on 2015 January (Atel#6917). Thus this is the second observation of the apastron outburst from this source. We encourage multi-wavelength observations to investigate this peculiar brightening.
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).