MAXI/GSC observation of the onset of a giant outburst from Be/X-ray binary pulsar GRO J1008-57

ATel #13750; M. Nakajima, H. Negoro (Nihon U.), T. Mihara (RIKEN), M. Aoki, K. Kobayashi, R. Takagi, K, Asakura, K, Seino, S. Mokumoto (Nihon U.), C. Guo, Y. Zhou, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, R. Adachi, M. Niwano (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka (JAXA), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC)
on 22 May 2020; 01:41 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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A rapid brightening of the Be/X-ray binary pulsar GRO J1008-57 triggered on the MAXI/GSC nova-alert system (Negoro et al. 2016) on 2020-05-17 (MJD 58986). NICER observation about this outburst is reported in ATel #13749. After the last periastron passage on 2020 March, this source had been in the flaring state with the average flux of ∼40 mCrab for 2 months (ATel #13544). On 2020-05-06 (MJD 58975), the flux dropped once and showed an abrupt brightening after it. In the recent 10 days, the source 4-10 keV flux increased with a rate of ∼0.020 photons/cm²/sec/day. These X-ray behavior is very similar to the beginning of the 2012 giant outburst of the source (Atel#4355,#4561,#4573,#4577). See the figure for a comparison of the current and 2012 outbursts, http://maxi.riken.jp/news/en/#news-1252. Assuming that this brightening traces the 2012 giant outburst, the current outburst will reach its maximum of ∼500 mCrab at the end of May.

We encourage multi-wavelength observations to understand the physical mechanisms of the giant outburst.

The latest X-ray light-curve can be checked at the following pages;
MAXI ( http://maxi.riken.jp/star_data/J1009-582/J1009-582.html ),
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 ).