TITLE: GCN CIRCULAR NUMBER: 39224 SUBJECT: EP250207a: EP-WXT detection and XRT follow-up observation DATE: 25/02/08 05:14:40 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

D. Y. Li, M. J. Liu, J. An, S.Y. Fu, S. Q. Jiang, X. Liu, D. Xu, Z. P. Zhu, M. H. Zhang (NAOC, CAS), Q. C. Shui (IHEP, CAS), W. D. Zhang (NAOC, CAS) report on behalf of the Einstein Probe team:

We report on the detection of an X-ray transient by EP-WXT, designated EP250207a, which triggered the on-board processing unit at 2025-02-07T15:51:17 (trigger ID: 01709131361). The WXT position of the source is R.A. = 356.902 deg, DEC = 27.027 deg (J2000) with an uncertainty of 2.2 arcmin in radius (90% C.L. statistical and systematic). The average 0.5-4 keV spectrum can be fitted with an apec model with a temperature of 3.1 (-1.2, +5.9) keV. No significant absorption is required during the spectral fitting. The derived average 0.5-4 keV flux is 2.9 (-0.7, +0.9) x10^-10 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters. We performed a target of opportunity observation with Swift. The Swift XRT observation began at 2025-02-07T17:59:50 (UTC) with an exposure time of 1716 seconds in the Photon Counting mode, about 2 hours after the burst detected by EP-WXT. A bright X-ray source, which is spatially consistent with the low-mass star 2MASS J23473680+2702068 (distance ~ 21.3 pc), was detected within the WXT error circle. The X-ray spectrum can be fitted with two apec components, with temperature of 0.75 (-0.3, +0.2) keV, and 2.6 (-0.6, +1.3) keV, respevtively. No significant absorption is required during the spectral fitting. The derived 0.3-10 keV flux is 9.2 (-1.0, +1.1)x10^-12 erg/s/cm2, corresponding to a luminosity of around 5x10^29 erg/s. We suggest this transient is a stellar flare from this low-mass star. We thank the Neil Gehrels Swift team for making the quick follow-up observation possible. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

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