XRISM/Xtend Transient Search (XTS) detected an X-ray flare possibly from 1eRASS J053652.4+211435

ATel #18112; A. Yoshimoto (NWU), Y. Maeda (JAXA), T. Hakamata, Y. Kanemaru, A. Miyamoto, Y. Omiya, Y. Uchida, T. Yoneyama (JAXA), M. Audard (U. de Geneve), E. Behar (Technion), K. Fukushima (TUS), K. Hasegawa (Kyoto U.), H. Iinuma (Tohoku U.), S. Inoue (Kyoto U.), Y. Ishihara (Chuo U.), N. Ishii (Kyoto U.), S. Ishiwata (Chuo U.), C. Kang (Ehime U.), R. Kawada (Kyoto U.), M. Kikuchi (Ehime U.), T. Kiyomoto (Saitama U.), T. Kohmura (TUS), H. Kuramoto (UOsaka), W. Lee (Kyoto U.), H. Matsumoto (UOsaka), A. Miyamoto (UOsaka), M. Mizumoto (UTEF), Y. Motogami (Saitama U.), H. Murakami (TGU), T. Narita (RIKEN), M. Nobukawa (NUE), H. Noda (Tohoku U.), S. Ogawa (TUS), K. Sakamoto (Kyoto U.), Y. Sakamoto (Tohoku U.), M. Shidatsu (Ehime U.), H. Takahashi (Hiroshima U.), T. Takagi (Ehime U.), S. Takatsuka (UOsaka), F. Takisawa (Ehime U.), Y. Terada (Saitama U., JAXA), Y. Terashima (Ehime U.), Y. Tsuboi (Chuo U.), A. Uchida, H. Uchida (JAXA), T. Usuki (Ehime U.), M. Yoshimoto (Ehime U.)
on 19 Oct 2026; 13:01 UT
Credential Certification: Tomokage Yoneyama (tyoneyama263@g.chuo-u.ac.jp)

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XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray source XRISM J0537+2115 on 2026-10-08 TT. The source position was determined to be (R.A., Dec.) = (84.223, 21.247), with a systematic error of ∼ 40 arcsec. A plausible counterpart is an X-ray source 1eRASS J053652.4+211435 located ∼ 20 arcsec apart from the position of XRISM J0537+2115, respectively. All statistical uncertainties in this report are provided at the 90% confidence level.

Because of gaps in the XRISM time coverage, we could not robustly determine the flare's start and peak times. The peak time is roughly constrained to 2026-10-08 01:07 – 01:57 TT, followed by an exponential decay with an e-folding timescale of approximately ∼ 2200 s. The time-averaged spectrum was fitted with an absorbed apec model, yielding NH < 3 × 10²⁰ cm^-2 and kT = 0.75 ^+0.14_-0.17 keV. The power-law model also provided a statistically comparable fit. The flux was calculated as 5.8 ^+0.8_-3.2 × 10^-13 erg s^-1 cm^-2 (0.4 – 10.0 keV). A systematic error of roughly 20% should be added to the statistical error. Corresponding luminosity was 3.2 ^+0.4_-1.7 × D_68pc × 10²⁹ erg s^-1 by assuming the distance to XRISM J0537+2115 of D_68pc.