TITLE: GCN CIRCULAR NUMBER: 41277 SUBJECT: EP250807a: EP-WXT detection of a fast X-ray transient DATE: 25/08/08 07:28:10 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

EP250807a: EP-WXT detection of a fast X-ray transient

Y. Wang (PMO, CAS), Y. Q. Zhao (USTC,PRIC), H. Q. Cheng, Z. X. Ling (NAOC) 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 EP250807a. The transient triggered EP-WXT (ID: 15309198471) at 2025-08-07T23:33:02 (UTC). The WXT position of the source is R.A. = 281.311 deg, DEC = 14.383 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The duration of the flare is about 250 seconds, with a peak flux of 2.08 x 10^-8 ergs/s/cm^2. The WXT spectrum during the flare can be fitted with an absorbed power-law model, yielding best-fit absorption column density of 0.80 (-0.26, +0.27) x 10^22 cm^-2 and photon index of 1.99 (-0.11, +0.14). The 0.5-4 keV unabsorbed flux is 7.14 (-1.59, +2.71) x 10^-9 ergs/s/cm^2.

Follow-up observations with EP-FXT are scheduled.

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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