MAXI/GSC Observation of an X-ray Outburst from the Be/X-ray Binary Pulsar 1A 0535-66 (1A 0538-66)
ATel #17815; M. Nakajima (Nihon U.), T. Mihara (RIKEN), M. Sugizaki (Kanazawa U.), H. Negoro, K. Saito, T. Yokoyama, M. Wada (Nihon U.), T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, 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. Kurihara (JAXA), Y. Ueda, K. Fujiwara, S. Kobayashi (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), S. Ogawa (TUS)
on 26 May 2026; 14:00 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 on an X-ray brightening from the Be/X-ray binary pulsar 1A 0535-66 (also known as 1A 0538-66) on 2026 May 21 (MJD 61181; see the MAXI alert page at https://maxi.riken.jp/alert/novae/1181387701/1181387701.htm ). This marks the third firm detection of this source by MAXI/GSC. The previous activities were recorded on 2025 September 30 (MJD 60948; ATel #17455, #17467) and 2026 February 27 (MJD 61098, ATel #17705). Similar to the previous events, the current outburst exhibited a fast-rise and exponential-decay (FRED) profile. To obtain a correct light curve in a crowded region, we performed a 'image-fit' analysis (Morii et al. 2016) following the procedure used in our previous report (ATel #17705). We extracted the light curve while avoiding contamination from the nearby source LMC X-4. By fitting the obtained a 90-minute binned light curve in the 4-10 keV band with a FRED model, we determined the parameters of the outburst (onset time, peak time, and peak flux). The outburst onset time was 18:01 UT on 2026 May 20 (MJD 61180.751) and the peak time was 13:12 UT on May 21 (MJD 61181.550). 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), these onset and peak times correspond to the orbital phases of 0.952 and 0.000, respectively. The derived peak flux in the 4-10 keV band is 0.126 ± 0.016 photons s-1 cm-2 (104 ± 13 mCrab). This corresponds to the X-ray luminosity of ∼3 × 1038 erg s-1, assuming a Crab-like single power-law spectrum and a distance of 50 kpc. After the peak, the outburst exponentially decayed with τ = 0.57 ± 0.15 d. The obtained outburst parameters are similar to those of the outburst observed in 2025 September (ATel #17455, #17467). A comparison of three outburst profiles is available at the following link.
A comparison of three outburst profiles.
