MAXI observation of a bright outburst from a Be/X-ray binary 1A 0538-66 around September 30, 2025.
ATel #17467; M. Nakajima (Nihon U.), M. Sugizaki (Kanazawa U.), T. Mihara (RIKEN), H. Negoro, K. Takagi, H. Takahashi, H. Nishio (Nihon U.), T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo U.), M. Shidatsu, C. Kang, T. Nakamoto, M. Uenishi, T. Usuki, S. Yatsuzuka (Ehime U.), I. Takahashi, Y. Yatsu (Science Tokyo), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda, K. Fujiwara (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), K. Yamaoka (Nagoya U.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), S. Yamada (Tohoku U)
on 1 Nov 2025; 03:02 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
Referred to by ATel #: 17705
1A 0538-66 is a Be/X-ray binary hosting a ∼69 ms pulsar located in the Large Magellanic Cloud (LMC). Recently, Ducci et al. (ATel #17455) reported that the object exhibited a bright outburst with a luminosity reaching the super-Eddington level (∼1.5×1039 erg s-1 in the 0.2-12 keV band) on 2025 September 30 (MJD 60948). MAXI/GSC also detected this event, and the data revealed that the outburst lasted for about two days. This is the first firm detection of 1A 0538-66 by MAXI/GSC since the mission began its in-orbit operations in 2009.
Because 1A 0538-66 is located only 0.5 degree away from LMC X-4, the standard data analysis (e.g., MAXI on-demand process; https://maxi.riken.jp/mxondem ) cannot be applied without the contamination from the nearby bright source. Therefore, we performed an "image-fit" analysis (e.g. Kudo et al. 2025; ATel #16975) to obtain the 4-10 keV light curve. The resulting light curve, binned at 0.0625 days, shows a short-duration outburst characterized by a fast rise and exponential decay, with the best-fit parameters: Tstart = 60948.09+0.10-0.12(MJD), Tpeak = 60948.61+0.14-0.11 (MJD), and τ = 0.75+0.19-0.16 days. (The errors represent the 90% confidence range.) Assuming the orbital ephemeris of Porb = 16.64002 ± 0.00026 d and T0 = 55673.71 ± 0.05 (MJD) given by Rajoelimanana et al. (2017) and updated by Ducci et al. (2022), Tstart and Tpeak correspond to orbital phases of 0.97 and 0.00, respectively. The observed peak flux of 0.184 ± 0.062 photons s-1 cm-2 (147 ± 50 mCrab) at MJD 60948.5018 (2025 Sep 30 12:02 UT) corresponds to the 4-10 keV luminosity of 5.1×1038 erg s-1, assuming the Crab-like single power-law spectrum.
We also analyzed the X-ray spectrum obtained from the MAXI on-demand process. Since the source region includes LMC X-4, data from the pre-outburst period (MJD 60676 [2025 Jan 1] - 60887 [2025 Jul 31]) were used as the background. The 2-10 keV spectrum can be fitted with a single power-law model, yielding the best-fit photon index of 1.6 ± 0.2 with an absorption column density fixed at 1.5×1021 cm-2 as adopted in ATEL#17455. The absorbed 2-10 keV flux is estimated to be (1.3 ± 0.1)×10-9 erg cm-2 s-1, corresponding to a luminosity of (4.0 ± 0.4)×1038 erg s-1.
