TITLE: GCN CIRCULAR NUMBER: 40017 SUBJECT: EP250402a: Einstein Probe detected of a fast X-ray transient DATE: 25/04/03 06:33:31 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

R.-Z. Li (YNAO, CAS), H. Q. Cheng (NAO, CAS), Q. C. Zhao, Z. H. Yang (IHEP, CAS), Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:

We report on the detection of a fast X-ray transient EP250402a (trigger ID: 01709133860) at 2025-04-02T12:29:06.850 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The position of the source is R.A., DEC. = 172.168, -47.290 deg (J2000) with an uncertainty of 2.5 arcmin (radius, 90% C.L. statistical and systematic). The transient event lasted for about 80 seconds, with a peak 0.5-4 keV flux of 5.7 x 10^-9 erg/s/cm^2.

The 0.5-4.0 keV spectrum can be fitted with an absorbed powerlaw model, with the column density nH fixed at the Galactic value of 1.43 x 10^21 cm^-2 and a photon index of 1.3 (+0.7/-0.7). The unabsorbed 0.5-4.0 keV flux is 1.2 (+0.6, -0.4) x10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

Follow-up observations by EP-FXT are prepared.

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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