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MAXI/GSC observation of the onset of the outburst of the Be/X-ray binary pulsar 1A 1118-61

ATel #17613; M. Nakajima (Nihon 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, 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)
on 20 Jan 2026; 02:34 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 17615, 17616, 17626, 17667, 17690

We report on the onset phase of the outburst of the Be/X-ray binary pulsar 1A 1118-61 observed with MAXI/GSC. An X-ray brightening of 1A 1118-61 was first triggered by Swift/BAT observations on 2026 January 17 (MJD 61057; GCN Circulars 43435 and 43442 ; ATel #17608). The latest day-averaged 15-50 keV flux observed on 2026 January 18 (MJD 61058) reached 0.357 ± 0.018 counts cm^-2 s^-1 (1.64 ± 0.08 Crab), which is approximately three times higher than the peak flux of the previous outburst in 2009 (ATel #1896, #1949). Although Swift/BAT has monitored the abrupt brightening over the past two days, the data between 2026 January 10 (MJD 61050) and 2026 January 16 (MJD 61056) are not available in the public Swift/BAT light curve. The MAXI/GSC observations complement the onset phase of the current outburst. We obtained the 4-10 keV light curve using the image-fit procedure (ATel #16975) as shown at
https://maxi.riken.jp/plot/comp2009-2026_asm_gscif.gif
According to our data, the present outburst started on 2026 January 12 (MJD 61052) and the flux increase follows a trend similar to that of the previous outburst in 2009. If the current brightening exhibits the same outburst profile, the flux is expected to peak around 2026-01-22 (MJD 61062). Follow-up observations are encouraged.