MAXI/GSC observation of a bright outburst from a Be/X-ray binary 1A 0538-66

ATel #17705; M. Nakajima, H. Negoro (Nihon U.), T. Mihara (RIKEN), M. Sugizaki (Kanazawa U.), K. Takagi, H. Nishio (Nihon U.), T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, D. Iijima, 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, S. Ogawa, 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)
on 3 Mar 2026; 02:26 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

Tweet

The MAXI/GSC nova alert system triggered on an X-ray transient source in the Large Magellanic Cloud (LMC) region on 2026 February 27 (MJD 61098). Assuming that the source flux was constant over the seven transits between 02:14 UT and 11:32 UT, we derived the source position as (R.A., Dec.) = (83.624 deg, -66.880 deg) = (05 34 29, -66 52 47) (J2000), with a statistical 90% C.L. elliptical error region with long and short radii of 0.22 deg and 0.21 deg, respectively. The roll angle of the major axis from the north direction is 135.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The derived position is consistent with that of the Be/X-ray binary pulsar 1A 0538-66. This is the second firm detection of this source by the MAXI/GSC observation ; the previous X-ray outburst was recorded on 2025 September 30 (MJD 60948; ATel #17455, #17467).

Since the bright X-ray source LMC X-4 is located in the vicinity of 1A 0538-66, we cannot extract the light curve of 1A 0538-66 using the standard data analysis (e.g., the MAXI on-demand process; https://maxi.riken.jp/mxondem) without contamination. Therefore, we applied an 'image-fit' analysis (Morii et al. 2016), following the procedure adopted in the previous report (ATel #17467), to derive the source light curve. The derived 90-min binned light curve in the 4-10 keV band shows that the current outburst started at 02:15 UT on February 27, 2026 (MJD 61098.094) and peaked at 03:50 UT on the same day (MJD 61098.160). The observed peak flux in 4-10 keV is 0.178 ± 0.034 photons s^-1 cm^-2 (147 ± 28 mCrab). Assuming the orbital ephemeris of P_orb = 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), the outburst onset and peak times correspond to orbital phases of 0.9842 and 0.9882, respectively. These outburst parameters - onset phase, peak phase, and peak flux - are consistent with those of the previous event (ATel #17467).

The remarkable feature of the current outburst is that the flux increased again at an orbital phase of 0.03 (MJD 61098.868) instead of following the fast-rise exponential-decay profile observed in the previous outburst (ATel #17467). The peak flux of this second episode reached approximately 200 mCrab at 09:14 UT on February 28 (MJD 61099.385). No significant spectral (color) changes were observed around the second peak. The 4-10 keV luminosity during the second peak exceeded that of the first peak. Assuming a Crab-like single power-law spectrum, the peak luminosity of the second episode is estimated to be approximately 8 × 10³⁸ erg s^-1, assuming a distance of 50 kpc.

The latest observation shows that the source flux in 4-10 keV band decreased to 0.067 ± 0.025 photons s^-1 cm^-2 (55 ± 21 mCrab) at 10:01 UT on 2026 March 1 (MJD 61100.418).

1A 0538+66 light curve