Possible first detection of X-ray burst from 4U 1543-624

ATel #11302; M. Serino (AGU), K. Hidaka (Miyazaki U.), H. Negoro (Nihon U.), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), T. Mihara, M. Sugizaki, S. Nakahira, W. Iwakiri, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), N. Kawai, S. Sugita, T. Yoshii, Y. Tachibana, S. Harita, Y. Muraki, K. Morita (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka, T. Hashimoto (AGU), H. Tsunemi, T. Yoneyama (Osaka U.), M. Nakajima, T. Kawase, A. Sakamaki, Y. Maruyama (Nihon U.), Y. Ueda, T. Hori, A. Tanimoto, S. Oda, T. Morita, S. Yamada (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki, H. Kawai, T. Sato (Chuo U.), M. Yamauchi, C. Hanyu (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), M. Shidatsu (Ehime U.)
on 15 Feb 2018; 05:16 UT
Credential Certification: Motoko Suzuki (motoko@crab.riken.jp)

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The MAXI/GSC nova alert system triggered a bright X-ray transient source at 00:09:17.00 UT on 2018 February 2. The position of the source was consistent with LMXB 4U 1543-624. The X-ray flux averaged over the scan was 938 +- 61 mCrab (4.0-10.0keV, 1 sigma error). There was no significant excess flux in the previous transit at February 1 22:36 UT and in the next transit at February 2 1:42 UT with an upper limit of 20 mCrab for each.

The light curves in 2-4, 4-10, 10-20 keV bands of the scan transit show that the spectrum of the source was hard in the beginning and then softened at the decay part. The time averaged spectrum is fit by blackbody model with kT = 1.8+-0.2 keV. Although we could not find any historical report of X-ray burst from this source, the observed features are consistent with a type-I X-ray burst. The 1s peak flux was 4+-1 photons cm^-2 s^-1 in 2-20 keV. Assuming the empirical maximum luminosity of X-ray bursts, 3.8 x 10³⁸ erg s^-1 (Kuulkers et al., 2003, A&A, 399, 663), we estimate the upper limit of the source distance as 9.2+-2.3 kpc. This is consistent with previous estimation of the distance of ~7 kpc (Wang & Chakrabarty, 2004, ApJL, 616, L139).