MAXI/GSC onset time of the X-ray outburst and detection of an X-ray burst from LMXB EXO 0748-676

ATel #16654; T. Mihara (RIKEN), M. Nakajima, H. Negoro (Nihon U.), W. Iwakiri (Chiba U.), M. Serino (AGU), Y. Kudo, H. Shibui, K. Takagi, H. Takahashi, K. Tatano, H. Nishio (Nihon U.), T. Kawamuro, S. Yamada, S. Wang, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu, H. Nishikawa, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima (Chuo U.), M. Shidatsu, Y. Niida (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 (Kyoto U.), M. Yamauchi, Y. Otsuki, T. Hasegawa, M. Nishio (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (NAOC) report on behalf of the MAXI team:
on 15 Jun 2024; 02:52 UT
Credential Certification: Tatehiro Mihara (mihara@crab.riken.jp)

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The MAXI/GSC nova alert system triggered an X-ray burst from the low-mass X-ray binary EXO 0748-676 at 2024-06-13 19:23 UT. This is the first detection of an X-ray burst from this source by MAXI/GSC since the start of the observation in 2009. The burst was short lasting about 10 seconds, and any obvious burst cooling was not observed. The X-ray spectrum extracted from the scan transit covering the entire burst can be reproduced by the blackbody model with a temperature of kT = 1.6 ± 0.2 keV (90% CL). The burst was shaped as a top-flat symmetric trapezoid rather than a fast-rise-exponential-decay. The peak flux was about 2 Crab in 4-10 keV, which corresponds to the luminosity of 2× 10³⁸ ergs/s, assuming a distance of 7.4 kpc.
According to the recent reports (GCN#36653, ATel#16646, #16648, #16649, #16651), EXO 0748-676 returned to an active state after 16 years of quiescence since 2008. Based on the MAXI/GSC monitor data, the flux started to rise around 60440 MJD (May 10, 2024). It reached about 15 mCrab around 60475 MJD (June 14, 2024). The average persistent X-ray luminosity in the recent 3 weeks is 1.5×10³⁶ ergs/s comparable to that of the previous active state.
The latest X-ray light-curve can be checked at the following page;
MAXI ( http://maxi.riken.jp/v7l3h/J0748-677/index.html ), and
Swift/BAT ( https://swift.gsfc.nasa.gov/results/transients/weak/EXO0748-676/ ).