TITLE: GCN CIRCULAR NUMBER: 42834 SUBJECT: EP251124b: EP-WXT detection of a fast X-ray transient DATE: 25/11/25 15:33:27 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

J. P. Feng, B. Zhang (USTC), X. X. Sun (NAO, CAS), Q. C. Liu (THU), D. F. Hu, Y. L. Hua (PMO, CAS), C. C. Jin (NAO, CAS), 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 EP251124b. The transient was detected by EP-WXT at 2025-11-24T17:23:02 (UTC). The WXT position of the source is R.A. = 93.493 deg, DEC = 52.694 deg (J2000) with an uncertainty of 2.6 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for ～240 seconds, the average 0.5-4 keV spectrum can be fitted with an absorbed power-law model, with fixed galactic nH of 1.51 x 10^21 cm^-2 and a photon index of 1.72 (-0.78, +0.83). The unabsorbed average 0.5-4 keV flux is estimated to be 2.97 (-0.95, +1.51) x 10^-10 erg/s/cm2. The peak flux reaches around 4.8e-9 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters.

The transient was discovered by the ground analysis of the delayed telemetry data. The follow-up observation of EP-FXT has been 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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