MAXI detection of a long X-ray burst possibly from MAXI J0911-655/Swift J0911.9-6452

ATel #13754; M. Nakajima (Nihon U.), S. Sugita, M. Serino (AGU), W. Iwakiri (Chuo U.), H. Negoro, M. Aoki, K. Kobayashi, R. Takagi, K, Asakura, K, Seino, S. Mokumoto (Nihon U.), T. Mihara, C. Guo, Y. Zhou, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, H. Nishida, 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 (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 23 May 2020; 06:15 UT
Credential Certification: Motoko Suzuki (motoko@crab.riken.jp)

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The MAXI/GSC nova alert system triggered on an X-ray burst from the position consistent with a globular cluster NGC2808 which is the host of an accreting millisecond X-ray pulsar MAXI J0911-655/Swift J0911.9-6452 (Serino et al. 2016, Kennea et al. 2016, Sanna et al. 2017). No type-I burst has been observed in this source so far. GSC observed the region around 17:04 and 18:37 on May 22 2020 UT. The spectra of the first and the second scan are fitted by an absorbed blackbody model with a temperature kT = 2.5 (+0.2, -0.2) and 0.8 (+0.3, -0.2) keV, respectively, where we assumed the hydrogen column density of 0.27 × 10²² cm^-2 (Sanna et al. 2017). Assuming a distance of 9.5 kpc (Watkins et al. 2015), the unabsorbed bolometric luminosity was decayed from about 4.0 ± 0.4 × 10³⁸ to 0.3 ± 0.1 × 10³⁸. Since Eddington limit for a He type-I X-ray burst is 3.8 × 10³⁸ erg/s (empirically derived by Kuulkers et al. 2003), the luminosity at the first MAXI scan reached Eddington limit. The e-folding time of the burst is estimated as 36 min suggesting an intermediate duration burst.

Kennea et al. 2016, ATel #8884
Kuulkers et al. 2003, A&A, 399, 663
Sanna et al. 2017, A&A, 598, A34
Serino et al. 2016, ATel #8872
Watkins et al. 2015, ApJ, 812, 149