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MAXI/GSC detections of a new superburst from Aql X-1

ATel #14079; W. Iwakiri (Chuo U.), M. Serino (AGU), H. Negoro, M. Nakajima, M. Aoki, K. Kobayashi, R. Takagi, K, Asakura, K, Seino (Nihon U.), T. Mihara, C. Guo, Y. Zhou, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, H. Nishida, K. Komachi, A. Yoshida (AGU), Y. Tsuboi, R. Sasaki, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, R. Adachi, M. Niwano (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, U. Goto, R. Uematsu (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC)
on 9 Oct 2020; 10:12 UT
Credential Certification: Wataru Iwakiri (wataru.iwakiri@riken.jp)

Subjects: X-ray, Neutron Star, Transient

Referred to by ATel #: 14437, 16826

We report a superburst from Aql X-1 found in MAXI/GSC observations on 2020 October 8 (MJD 59130). The detection of this event was 16:59 UT and lasted for at least 6 hours in the total five scan transits of MAXI. The light curve was shaped as a fast-rise and an exponential-decay with the e-folding decay time of 2.3 (+0.9, -0.6) hours.
Assuming that the persistent emission did not change during the burst, we extracted the spectrum from 4 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 first three scan transits at 16:59, 18:32 and 20:05 UT is well fitted by an absorbed blackbody model with temperatures of kT = 2.0 (+/- 0.1), 1.4 (+0.1, -0.2) and 1.3 (+/- 0.2) keV, respectively. Here, the hydrogen column density was fixed at 6 x1021 cm-2 (Bult et al. 2018). The bolometric flux declined from 2.9 (+0.3, -0.2) x10-8 to 0.8 (+0.1, -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 Aql X-1 with MAXI after the first one on 2013 Jul 20 (Serino et al. 2016).

Bult et al. 2018, ApJL, 859, L1
Serino et al. 2016, PASJ, 68, 95