MAXI/GSC detection of X-ray outburst from Be/X-ray binary pulsar Cep X-4

ATel #11791; M. Nakajima, H. Negoro (Nihon U.), T. Mihara (RIKEN), W. Iwakiri (Chuo U.), A. Sakamaki, W. Maruyama (Nihon U.), S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), N. Kawai, M. Sugizaki, Y. Tachibana, K. Morita (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), Y. Ueda, A. Tanimoto, T. Morita, S. Yamada (Kyoto U.), Y. Tsuboi, R. Sasaki, H. Kawai, T. Sato (Chuo U.), H. Tsunemi, T. Yoneyama (Osaka U.), M. Yamauchi, K. Hidaka, S. Iwahori (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), M. Shidatsu (Ehime U.)
on 28 Jun 2018; 08:02 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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MAXI/GSC confirmed the X-ray brightening from the Be/X-ray binary pulsar Cep X-4(= 4U2135+57 and Ginga 2138+56) reported by Jenke et al. (2018, Atel#11780). The latest one-day averaged X-ray flux in 4-10 keV band reached 0.055 ± 0.015 ph/s/cm² (48 ± 13 mCrab) on 2018-06-27 (MJD 58296). Due to the Cep X-4 region was out of FOV until 2018-06-24 (MJD 58293), the onset of this outburst was not able to be observed by MAXI/GSC. The 15-50 keV data obtained by BAT onboard the Neil Gehrels Swift observatory indicated that the current outburst started from 2018-06-20 (MJD 58289). Assuming that the present outburst traces the same profile of the previous outbursts in 2009 (Atel #1956, #1972) and 2014 (Atel #6212, #6243, #6265), the maximum flux is expected to be around 2018-07-03 (MJD 58301). Orbital ephemeris have not been reported on this source. Therefore, we can not categorize this event as a normal outburst or a giant one. We encourage multiwavelength observations to reveal the nature of this source.

The latest source flux can be checked at the following pages;
MAXI(http://maxi.riken.jp/star_data/J2139+569/J2139+569.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/webpage_oneplot.php).