MAXI/GSC detection of an X-ray outburst from the Be/X-ray binary pulsar Cep X-4 (Ginga 2138+56)

ATel #16088; M. Nakajima (Nihon U.), Y. Niida (Ehime U.), T. Mihara (RIKEN), H. Negoro, K. Kobayashi, M. Tanaka, Y. Soejima, Y. Kudo (Nihon U.), T. Kawamuro, S. Yamada, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Hiramatsu, H. Nishikawa, A. Yoshida (AGU), Y. Tsuboi, S. Urabe, S. Nawa, N. Nemoto (Chuo U.), M. Shidatsu (Ehime U.), I. Takahashi, M. Niwano, S. Sato, N. Higuchi, Y. Yatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, T. Kurihara (JAXA), Y. Ueda, K. Setoguchi, T. Yoshitake, Y. Nakatani (Kyoto U.), M. Yamauchi, Y. Hagiwara, Y. Umeki, Y. Otsuki (Miyazaki U.), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC), W. Iwakiri (Chiba U.)
on 17 Jun 2023; 13:16 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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The MAXI/GSC nova alert system triggered an X-ray transient source on 2023 June 15 (MJD 60111). Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (325.186 deg, 56.849 deg) = (21 40 44, +56 50 56) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.18 deg and 0.18 deg, respectively. The roll angle of the long axis from the north direction is 90.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). This position is consistent with the Be/X-ray binary pulsar Cep X-4 (Ginga 2138+56). This is the third detection of this source by MAXI/GSC after 2014 June (ATel #6212) and 2018 June (ATel #11791). The X-ray flux averaged over the scans from June 14 22:25 UT to June 16 20:49 UT was 56 ± 7 mCrab (4.0-10.0keV, 1 sigma error). Since the source was in the FOV of the degraded camera till June 14, the onset date of the current outburst is unclear. The current X-ray outburst is also observed by Swift/BAT and Fermi/GBM. According to the data in the 15-50 keV band, this outburst began on May 26 (MJD 60090), and the flux increased slowly with the rate of 1.3e-3 count/cm^2/s/day. The day-averaged flux is 93 ± 25 mCrab on June 15 (MJD 60110). This flux level is comparable to the previous outburst peaks observed in 2009 (ATel #1956), 2014, and 2018. We encourage multi-wavelength observations to reveal the nature of this source.

The latest X-ray light-curve can be checked at the following pages;
MAXI ( http://maxi.riken.jp/pubdata/v7lrkn/J2139+569/index.html ),
Swift/BAT ( https://swift.gsfc.nasa.gov/results/transients/weak/Ginga2138p56/ ),
Fermi/GBM ( https://gammaray.nsstc.nasa.gov/gbm/science/pulsars/lightcurves/cepx4.html ), and
the BeXRB monitor page ( http://integral.esac.esa.int/bexrbmonitor/Plots/sim_plot_LSV+4417.html ).