MAXI/GSC detection of a possible new transient MAXI J1609-487

ATel #18123; S. Ogawa (TUS), M. Serino, Y. Kawakubo (AGU), H. Negoro, M. Nakajima, K. Saito, T. Yokoyama, M. Wada (Nihon U.), T. Mihara, A. Aoyama, M. Ota, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, 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)
on 29 Oct 2026; 02:42 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Motoko Suzuki (motoko@crab.riken.jp)

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This is a cross-posting of GCN 46074. The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 16:32:33 UT on 28 October 2026. The source was detected in two transit scans at 16:32 UT and 18:05 UT on October 28, with X-ray fluxes of 382 +- 42 mCrab and 531 +- 49 mCrab (4.0-10.0keV, 1 sigma error), respectively. We obtain the source position at (242.237 deg, -48.765 deg) = (16 08 56, -48 45 54) (J2000) with a circular statistical error region of radius 0.12 deg at the 90% confidence level. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). There was no significant excess flux in the previous transit at 14:59 UT and in the transit at 19:38 UT with an upper limit of 20 mCrab for each. We tentatively name the source MAXI J1609-487 because the position of the source is close to the galactic plane and the behavior of the source is different from a typical GRB.