MAXI/GSC detection of a superburst from 4U 1608-52

ATel #17097; M. Serino (AGU), W. Iwakiri (Chiba U.), H. Negoro, M. Nakajima, Y. Kudo, H. Shibui, K. Takagi, H. Takahashi, K. Tatano, H. Nishio (Nihon U.), T. Mihara, S. Yamada, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu, H. Nishikawa, Y. Kondo, S. Sasao, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima (Chuo U.), M. Shidatsu, Y. Niida, C. Kang, T. Nakamoto (Ehime U.), I. Takahashi, M. Niwano, N. Higuchi, Y. Yatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda, Y. Okada, K. Fujiwara (Kyoto U.), M. Yamauchi, Y. Otsuki, T. Hasegawa, M. Nishio (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), T. Kawamuro (Osaka U.) on behalf of the MAXI team
on 21 Mar 2025; 14:55 UT
Credential Certification: Motoko Suzuki (motoko@crab.riken.jp)

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We report a superburst from 4U 1608-52 found in MAXI/GSC observations on March 19, 2025 (MJD 60753). The detection of this event was 23:27 UT and lasted for at least 3 hours in the total three scan transits of MAXI. The light curve was shaped as a fast-rise and an exponential-decay with an e-folding decay time of 2.1 (+-0.1) hours.

Assuming that the persistent emission did not change during the burst, we extracted the spectrum from 3 consecutive scans just before the burst as the persistent emission and included it in the fitting of the burst spectrum extracted from each scan.

Each spectrum of the three scan transits at 23:27, 00:59 on March 20 and 02:32 UT is well fitted by an absorbed blackbody model with temperatures of kT = 1.7 (+0.1, -0.1), 1.3 (+0.1, -0.1) and 1.2 (+0.3, -0.2) keV, respectively. Here, the hydrogen column density was fixed at 1.0 x10²² cm^-2 (Bhattacherjee et al. 2024). The bolometric flux declined from 3.3 (+0.3, -0.3) x10^-8 to 0.7 (+0.2, -0.2) x10^-8 erg s^-1 cm^-2 over the course of 3 hours. A long decay time of the burst (more than an hour) and cooling along with the decay clearly indicate a superburst. This is the second detection of a superburst from 4U 1608-52 with MAXI after the first one on July 16, 2020.

Bhattacherjee et al. 2024, ApJ, 971, 154.