MAXI/GSC detection of a long X-ray burst from 4U 1820-30

ATel #14871; M. Serino (AGU), W. Iwakiri (Chuo U.), H. Negoro, M. Nakajima, K. Kobayashi, K. Asakura, K. Seino (Nihon U.), T. Mihara, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, K. Komachi, A. Yoshida (AGU), Y. Tsuboi, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, M. Niwano, R. Hosokawa (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, Y. Goto, R. Uematsu (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, Y. Nonaka, T. Sato, R. Hatsuda, R. Fukuoka (Miyazaki U.), T. Kawamuro (UDP/NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC)
on 24 Aug 2021; 12:44 UT
Credential Certification: Motoko Suzuki (motoko@crab.riken.jp)

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MAXI/GSC detected a bright X-ray burst from the position consistent with that of the low-mass X-ray binary 4U 1820-30 at 11:27 UT on 2021 August 23. The 4-10 keV X-ray flux averaged over the scan transit assuming a constant flux was 1868 (+104, -94) mCrab (1 sigma error). In the next scan transit at 13:01, the source flux was 486 +- 50 mCrab in the same energy band, which was slightly higher than the persistent level (~200 mCrab). There was no significant excess flux in the previous transit at 09:55 UT. Because a burst like profile was not seen during the scan transit at 11:27, it suggests that the burst had already started before the scan transit at 11:27. 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. The absorbed blackbody model fit to the spectra of the first and the second scan transits in the 2-20 keV band gives the blackbody temperature of 2.4 (+0.3, -0.2) keV and 1.6 (+0.5, -0.4) keV, respectively. Here, the hydrogen column density was fixed at 2.7 × 10²¹ cm^-2 (Krim et al. 2009, ATel #2071). Assuming source distance of 7.6 kpc (Kuulkers et al. 2003), the bolometric source luminosity at the first scan was 2.8 (+0.4, -0.3) × 10³⁸ erg/s, which was 74% of empirical Eddington limit for a He type-I X-ray burst (3.8 × 10³⁸ erg/s; Kuulkers et al. 2003). The e-folding decay time calculated from the fluxes of the two scans was about 45 min. Previously, long X-ray bursts with the similar duration from this source were reported in 1999 (Strohmayer & Brown, ApJ, 566, 1045, 2002) and 2010 (in 't Zand et al. 2011, ATel #3625).