TITLE: GCN CIRCULAR NUMBER: 42001 SUBJECT: GRB 250925A/EP250925a: EP-WXT detection DATE: 25/09/26 13:07:12 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn
H. Zhou, D. F. Hu (PMO, CAS), Z. H. Yang, Q. C. Zhao (IHEP, CAS), Y. L. Wang (NAO, CAS; ICE, CSIC-IEEC) and 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 EP250925a. This transient was found in the ground-processed data, and consistent with GRB250925A in terms of time and space (Dichiara et al., GCN 41985). The start time of EP250925a is around 2025-09-25T17:03:22 UTC. The ground-porcessed WXT location of GRB 250925A is R.A. = 341.205 deg, DEC = 46.564 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic), which is consistent with the localization reported by other groups (Dichiara et al., GCN 41985; Page et al., GCN 41988; Corcoran et al., GCN 41990; Bochenek et al., GCN 41991; Masi et al., GCN 41992; Méndez et al., GCN 41995; Castro-Tirado et al., GCN 41997; Liu et al., GCN 41998).
The analysis of the WXT data shows that the event lasted for about 200 seconds with a peak flux of 1.5 x 10^-8 erg/s/cm^2. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 2.58 x 10^21 cm^-2 and a photon index of 1.18 +/- 0.18. The derived average unabsorbed 0.5-4 keV flux is (5.1 +/- 0.5) x 10^-9 erg/s/cm^2.
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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