X-ray rebrightening of the X-ray transient MAXI J1609-487

ATel #18136; H. Negoro, M. Wada (Nihon U.), M. Serino (AGU), T. Mihara (RIKEN), M. Nakajima, K. Saito, T. Yokoyama (Nihon U.), A. Aoyama, M. Ota, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu, T. Kato, 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), and S. Ogawa (TUS)
on 1 Nov 2026; 04:23 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The MAXI/GSC nova alert system triggered on the X-ray rebrightening of the X-ray transient MAXI J1609-487 at 00:18 UT on October 31, 2026. The source flux had been almost steady at 33 +/- 6 mCrab in the 4-10 keV band from 05:43 on Oct. 30 to 00:18 on Oct. 31, after which it began to increase. This recent flux increase, however, requires caution because of the incomplete background subtraction and effective area correction near the detector edge.

MAXI J1609-487 (GRB 261028B) exhibited a bright X-ray flare(s) observed in two scan transits on Oct. 28 (Ogawa et al. ATel #18123). No significant flux was detected before or after the flare. We obtained a GSC spectrum during the flare on Oct. 28, and found that the spectrum was well represented by an absorbed power-law model with a photon index of 1.79^{+0.43}_{-0.37}, and an equivalent hydrogen column density of 2.3^{+3.7}_{-2.3}e22. All errors are given at the 90% confidence level. The average 2-10 keV flux was 8.5e-9 erg/cm^2/s.

In addition to the source position near the Galactic plain, (l, b) = (333.0 deg, 2.2 deg), the subsequent X-ray rebrightening and the power-law spectrum suggest that the source is a Galactic transient (e.g., a supergiant fast X-ray transient, SFXT), rather than a gamma-ray burst. Follow-up observations are strongly encouraged to reveal the nature of this interesting object.

MAXI/GSC light curve for MAXI J1609-487