MAXI/GSC observation of the onset of the X-ray outburst of the low mass X-ray binary Aql X-1
ATel #17657; M. Nakajima (Nihon U.), S. Yatsuzuka (Ehime U.), H. Negoro, K. Takagi, H. Takahashi, H. Nishio (Nihon U.), T. Mihara, 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 (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 9 Feb 2026; 09:35 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)
Subjects: X-ray, Binary, Neutron Star, Transient
The MAXI/GSC nova alert system triggered on an X-ray transient source on February 8, 2026. Assuming that the source flux was constant over the nine transits between 11:11 UT and 23:35 UT, we derived the source position as (R.A., Dec) = (287.905 deg, 0.620 deg) = (19 11 37, +00 37 12) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.44 deg and 0.36 deg, respectively. The roll angle of long axis from the north direction is 160.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). This position is consistent with that of the low-mass X-ray binary Aql X-1. The previous X-ray activity of this source was reported in 2024 (ATel #16829). The X-ray flux averaged over the nine scans was 34±6 mCrab (4.0-10.0keV, 1 sigma error).
In the next scan after 23:35 UT on 2026 February 8, a thermonuclear burst was detected at 23:56 on the same day. The burst lasted for approximately 20 s and exhibits the fast-rise exponential-decay profile with an e-folding time of ∼5 s. The burst spectrum is well fitted by an absorbed blackbody model with a temperature of kT = 1.8+0.5-0.4 keV and the hydrogen column density of N_H = 6×1021 cm-2 (fixed; Bult et al. 2018). The average burst flux derived from this model is 3.3×10-9 erg cm-2 s-1 in the 2-10 keV band. Follow-up observations are encouraged.
