TITLE: GCN CIRCULAR NUMBER: 41207 SUBJECT: EP250730a: Einstein Probe detection of an X-ray transient DATE: 25/07/31 03:50:45 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

B.-T. Wang (YNAO, CAS), Y. L. Wang (NAO, CAS; ICE, CSIC-IEEC), X.P. Xu, W. Yuan (NAO, CAS) on behalf of the Einstein Probe (EP) team:

We report on the detection of an X-ray transient by EP-WXT through ground data analysis, designated EP250730a (ObsID:11900327297). The transient was detected at 2025-07-30T06:06:09 (UTC).

The WXT position of the source is R.A. = 313.264 deg, DEC = -4.648 deg (J2000) with an uncertainty of 2.1 arcmin (radius, 90% C.L. statistical and systematic). The transient event lasted for about 250 seconds.

The average 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model, with nH fixed at the Galactic value of 6.0 x 10^20 cm^-2 and a photon index of 1.84 (-0.38, +0.41). The unabsorbed average 0.5-4 keV flux is estimated to be 1.93 (-0.39, +0.48)e-10 erg/s/cm2. The peak flux is about 2e-9 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters.

Further FXT observations 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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