MAXI/GSC detections of a new superburst from GS 1826-238

ATel #11422; W. Iwakiri(RIKEN), M. Serino(AGU), T. Mihara, M. Sugizaki, S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), N. Kawai, S. Sugita, T. Yoshii, Y. Tachibana, S. Harita, K. Morita (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka, T. Hashimoto (AGU), H. Tsunemi, T. Yoneyama (Osaka U.), H. Negoro, M. Nakajima, T. Kawase, A. Sakamaki, W. Maruyama (Nihon U.), Y. Ueda, T. Hori, A. Tanimoto, S. Oda, T. Morita, S. Yamada (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki, H. Kawai, T. Sato (Chuo U.), M. Yamauchi, C. Hanyu, K. Hidaka (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.),M. Shidatsu (Ehime U.) report on behalf of the MAXI team:
on 15 Mar 2018; 14:17 UT
Credential Certification: Wataru Iwakiri (wataru.iwakiri@riken.jp)

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We report the MAXI/GSC detection of a new candidate superburst from GS 1826-238. The event from GS 1826-238 was found in MAXI/GSC observations on 2018 Feb 12 (MJD 58161). No superburst was observed from this source yet. The event was detected at 18:34 UT and lasted for at least 3 hours (three scan transits for MAXI) with a fast-rise and exponential-decay light curve. The spectrum of each of the three scan transits is well fitted by an absorbed blackbody model with temperatures kT = 1.8 (+/- 0.2), 1.5 (+/- 0.2) and 1.3 (+0.3, -0.2) keV at 18:34, 20:07 and 21:39 UT, respectively. The hydrogen column density was fixed at 0.28 x10²² cm^-2 (Ono et al 2016, PASJ, 68, 14). The bolometric flux declined from 2.9 (+/- 0.4) x10^-8 to 1.5 (+/- 0.3) x10^-8 erg/s/cm² over the course of 3 hours. The e-folding decay time is 4.3 (+5.5, -1.5) hours. Since these results prove a long decay time of the burst (more than an hour) and cooling along the decay, we conclude that this is the first superburst observed from GS 1826-238.