MAXI/GSC detection of a renewed activity from the Be/X-ray binary pulsar 1A 1118-616 or a possible new X-ray transient MAXI J1122-616

ATel #17853; M. Nakajima, H. Negoro (Nihon U.), T. Mihara (RIKEN), 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, 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 22 Jun 2026; 06:49 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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The MAXI/GSC nova alert system triggered on a weak X-ray enhancement from the vicinity of the Be/X-ray binary pulsar 1A 1118-61 on 2026 June 17. Assuming that the source flux was constant over the 26 scan transits between 21:28 UT on 2026 June 18 (MJD 61209) and 16:52 UT on 2026 June 20 (MJD 61211), we obtain the source position at
(R.A., Dec) = (170.442 deg, -61.632 deg) = (11 21 46, -61 37 55) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.49 deg and 0.39 deg, respectively. The roll angle of long axis from the north direction is 73.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The determined position deviates by ∼0.3 deg from the known location of 1A 1118-616. Although the X-ray flux averaged over these scans was 31 ± 4 mCrab (4.0-10.0keV, 1 sigma error), there is a possible increasing trend of ∼9 mCrab day^-1.

The obtained position error circle contains the position of 1A 1118-61 which exhibited a giant X-ray outburst in 2026 January/February (ATel #17608, #17613, #17615, #17616, #17620, #17626, #17627, #17667, #17690). However, the flux of 1A 1118-61 during the onset phase of the last giant outburst increased rapidly and did not show such a moderate increase.

On the other hand, the relatively constant faint flux (∼20-30 mCrab) lasting for a week is similar to the low-state activity observed in another Be/X-ray binary pulsar, 2S 1417-624 (ATel #12841). Therefore, 1A 1118-616 could still be a candidate of this brightening.

The absence of periodic dip structures in the light curve also allows us to exclude the possibility of the contamination from the nearby source Cen X-3 which exhibits the periodic eclipse events synchronized with its orbital period of ∼2.08 day (Klawin et al. 2023).

Based on the above, the triggered event could be a weak X-ray activity of 1A 1118-616. The possibility of a new transient, however, cannot be excluded. Thus, we have tentatively named this transient MAXI J1122-616. Multi-wavelength follow-up observations are strongly encouraged to clarify the nature of this source.