MAXI/GSC detection of XTE J1810-189 in outburst

ATel #18101; 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, M. Serino, 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 12 Oct 2026; 01:42 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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XTE J1810-189 was discovered in March 2008 during the RXTE PCA galactic ridge scan survey (Markwardt & Swank ATel #1424), and was soon found to be a neutron star X-ray binary through the detection of an X-ray burst (Markwardt et al. ATel #1443). After MAXI began observations, the source exhibited outbursts in 2012-2013 (Negoro et al. ATel #4752; Kuulkers et al. ATel #4808), 2020 (Bozzo et al. ATel #13983; also see Manca et al. 2023, MNRAS 526, 1154), and 2023 (Kobayashi et al. ATel #16000). During the 2023 NICER observations, a 458.92 Hz burst oscillation was discovered (Varun et al. 2025, ApJ 995, 153). We note that the Swift/BAT and MAXI/GSC light curves show that the source exhibited successive or double-peaked outbursts in 2008, 2012-2013, and 2020.

Recently, on September 12, 2026, MAXI detected an X-ray burst likely originating from XTE J1810-189 (Negoro et al. ATel #18039; also see Lower et al. ATel #18040). Detailed MAXI light curve analysis shows a gradual increase in X-ray flux since the end of July this year. The observed count rates in the 2-4, 4-10, and 10-20 keV bands have remained nearly stable since September 27, at (1.20 +/- 0.16)e-2 c/cm2/s (~ 11 mCrab), (2.6 +/- 0.2)e-2 c/cm2/s (~ 22 mCrab), and (5.2 +/- 1.8)e-3 c/cm2/s (~18 mCrab), respectively. These values are relatively fainter than previous outburst peaks (e.g., 50-70 mCrab in the 4-10 keV band) except for the peak of 25 mCrab in 2008 (Weng et al. 2015, MNRAS 450, 2915).

Followup observations are encouraged.

MAXI data for XTE J1810-189