TITLE: GCN CIRCULAR NUMBER: 39531 SUBJECT: EP250228a: updates on the EP observations DATE: 25/02/28 15:49:46 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

H.Y. Liu (NAO, CAS), J.Q. Peng (IHEP, CAS), W. F. Wen(SZTU), J. H. Wu (GZHU), C.C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:

The X-ray transient EP250228a (GCN 39525) triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2025-02-28T03:59:09 (UTC) and an autonomous observation was performed by the Follow-up X-ray Telescope (FXT) around 45 s later. The on-ground analysis of the FXT data shows that an uncatalogued source was detected at R.A. = 103.6059, DEC = -41.5710 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with that reported by the Swift XRT Team (GCN 39527). Given that the localisation of this source is well aligned with the position of Gaia DR3 5563280076339409920, EP250228a is likely a stellar flare associated with this star. The FXT spectrum can be well fitted by a 4T apec model with kT= 6.6(+3.4/-1.3), 1.4(+0.69/-0.35), 0.63(+0.16/-0.23), and 0.17(+0.04/-0.06) keV, respectively, and the derived flux is around 1.87 (+0.09/-0.09) x 10^-11 erg/s/cm^2 in 0.5-10 keV. The uncertainties are at the 90% confidence level for the above parameters.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with onboard X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

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