MAXI/GSC observation of a major X-ray outburst from the Be/X-ray Binary Pulsar 1A 0535-66 (1A 0538-66)
ATel #18020; M. Nakajima (Nihon U.), H. Hiramatsu (AGU), H. Negoro, K. Saito, T. Yokoyama, M. Wada (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, A. Yoshida (AGU), Y. Tsuboi, Y. Ishihara, H. Ogaki (Chuo U.), M. Shidatsu, C. Kang, M. Uenishi, T. Usuki, S. Yatsuzuka, H. Aoe (Ehime U.), I. Takahashi, Y. Yatsu (Science Tokyo), S. Nakahira, S. Ueno, H. Tomida, M. Kurihara (JAXA), Y. Ueda, K. Fujiwara, S. Kobayashi (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), S. Yamada (Tohoku U), S. Ogawa (TUS)
on 29 Aug 2026; 02:00 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
The MAXI/GSC nova alert system triggered an alert for an X-ray outburst from the Be/X-ray binary pulsar 1A 0538-66 at 12:35 UT on 2026 August 28 (MJD 61280). Prior to this trigger, the onset of the current outburst had already been detected in the previous MAXI scan at 11:02 UT. Using the orbital ephemeris of Porb = 16.64002 ± 0.00026 d and T0 = 55673.71 ± 0.05 (MJD), determined by Rajoelimanana et al. (2017) and updated by Ducci et al. (2022), the onset occurred at an orbital phase of 0.9437. Compared with previous 21st-century outbursts (ATel #17467, #17705, #17815), the current event began at an earlier orbital phase. Following the onset, the 4-10 keV flux of the source has been increasing at a rate of ∼0.5 counts s-1 cm-2 day-1. Preliminary analysis reveals that the 4-10 keV flux, averaged over 3 scans obtained by GSC5 between 19:07 UT and 22:13 UT on August 28 (MJD 61280), reached 301 ± 22 mCrab, highest flux ever recorded from this source in MAXI observations. Assuming a source distance of 50 kpc, this flux corresponds to an X-ray luminosity of 1.1 × 1039 erg s-1. Prior to this event, XMM-Newton and MAXI had detected 4 outbursts from this source: on 2025 May 20 (MJD 60815; unreported), 2025 September 30 (MJD 60948; ATel #17455, #17467), 2026 February 27 (MJD 61098; ATel #17705), and 2026 May 21 (MJD 61181; ATel #17815) to date.
Multi-wavelength follow-up observations are strongly encouraged to investigate the nature of this peculiar object, which exhibits characteristics of both SFXTs and ULX pulsars.
the MAXI alert information
