MAXI/GSC observation of the onset of a possible giant outburst from Be/X-ray binary pulsar EXO 2030+375

ATel #14809; M. Nakajima (Nihon U.), T. Mihara (RIKEN), H. Negoro, K. Kobayashi, K. Asakura, K. Seino (Nihon U.), T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, K. Komachi, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, M. Niwano, R. Hosokawa (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, Y. Goto, R. Uematsu (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, Y. Nonaka, T. Sato, R. Hatsuda, R. Fukuoka (Miyazaki U.), T. Kawamuro (UDP/NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC)
on 26 Jul 2021; 11:58 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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On 2021-07-20 (MJD 59415), the MAXI/GSC nova-alert system (Negoro et al. 2016) triggered on a faint X-ray activity from the Be/X-ray binary pulsar EXO 2030+375. EXO 2030+375 has been showing the normal outburst at every periastron passage (~46 days periodicity; Wilson et al. 2008) after the orbital phase shift of the outburst in 2016 (See Laplace et al. (2017) for long-term light curve). Since the regular normal outburst lasts for ~10 days, the source is in quiescent state in the other orbital phase. However, after the last periastron passage (MJD 59384), the source flux did not return to the quiescent state, and stayed constant at ~20 mCrab (4-10keV). This low flux state lasted for ~20 days until 2021-07-23 (MJD 59418, or orbital phase of ~0.7), and then, the source flux started to brighten with the increase rate of 10 mCrab/day. The onset phase of the current outburst is ~10 days (or 0.2 orbital phase) earlier than that of the nominal outburst. In addition, we notice that the current outburst nicely traces the previous giant outburst in 2006 (see, http://maxi.riken.jp/figure/exo2030/exo2030_lc_gobprofile.png ). Therefore, another giant outburst would be taking place. The next periastron passage is 2021-08-03 (MJD 59429).
We encourage multi-wavelength observations to reveal 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/pubdata/v7lrkn/J2032+376/index.html ),
Swift/BAT ( https://swift.gsfc.nasa.gov/results/transients/EXO2030p375/ ),
Fermi/GBM ( https://gammaray.nsstc.nasa.gov/gbm/science/pulsars/lightcurves/exo2030.html ), and
the BeXRB monitor page ( http://integral.esac.esa.int/bexrbmonitor/Plots/sim_plot_EXO2030+375.html ).