TITLE: GCN CIRCULAR NUMBER: 41619 SUBJECT: EP250831a: EP-WXT detection of a fast X-ray transient DATE: 25/08/31 15:00:03 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

J.Yang (ZZU), X. Tian (GXU), D. Y. Li, Y. Liu (NAO, CAS) report on behalf of the Einstein Probe (EP) team:

We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250831a (trigger ID: 11916651030). The WXT position of the source is R.A. = 42.951 deg, DEC = -36.175 deg (J2000) with an uncertainty of 2.9 arcmin in radius (90% C.L. statistical and systematic). The transient started at 2025-08-31T09:49:12 (UTC) and lasted for about 300 seconds seen from the WXT light curve. The 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model with Galactic hydrogen column density fixed to 1.9x10^20 cm^-2 and a photon index of 1.07(-0.49, +0.52). The average unabsorbed 0.5-4 keV flux is approximately 1.4 x 10^-10 erg/s/cm^2. The peak flux is estimated to be 6.8 x 10^-10 erg/s/cm^2.

Follow-up observations with the Follow-up X-ray Telescope (FXT) on board EP have been scheduled, and further information will be updated when the telemetry data received.

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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