TITLE: GCN CIRCULAR NUMBER: 39165 SUBJECT: EP250205a/GRB 250205A: Einstein Probe observation DATE: 25/02/06 09:14:37 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

Z. Y. Liu (USTC), M. H. Zhang, M. J. Liu, H. N. Yang, W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team

We report on the detection of an X-ray transient, designated EP250205a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, which triggered the on-board processing unit at 2025-02-05 21:32:08 (UTC) (trigger ID: 01709131283). The source position is R.A. = 113.522 deg, DEC = 32.363 deg (J2000) with an uncertainty of about 2.5 arcmin (radius, 90% C.L. statistical and systematic). An autonomous follow-up observation of EP250205a was performed by the Follow-up X-ray Telescope (FXT) on board EP, 256s after the WXT trigger. An uncatalogued X-ray source was detected at R.A. = 113.509 deg, DEC = 32.373 deg (J2000) with an uncertainty of less than 10 arcsec (radius, 90% C.L. statistical and systematic), consistent with the WXT position of EP250205a.

The 0.5-4.0 keV WXT spectrum can be fitted with an absorbed power law with a fixed Galactic absorption with column density 4.4 x 10^20 cm^-2 and an intrinsic absorber with a redshift of 3.55. The fitted equivalent hydrogen column density of the intrinsic absorber is 6.5(+9.8/-6.5) x 10^22 cm^-2, the photon index 2.5(+1.7/-1.2), and the unabsorbed 0.5-4 keV flux 4.2 (+1.1/-1.1) x 10^-10 erg/s/cm^2.

The average 0.5-10 keV spectrum of the follow-up observation obtained by EP-FXT can be fitted with an absorbed power law with a photon index of 2.82 (+0.06/-0.04) (with a galactic column density fixed at 4.4 x 10^20 cm^-2), yielding an average unabsorbed 0.5-10 keV flux of 3.65 (+0.07/-0.09) x 10^-11 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

EP250205a is spatially and temporally consistent with GRB 250205A (Saccardi et al. GCN 39154) at a redshift of 3.55 (de Ugarte Postigo et al. GCN 39160). EP250205a was first detected by WXT at 2025-02-05T21:31:28 (UTC), about 410 seconds after the SVOM/ECLAIRs trigger time (2025-02-05T21:24:38), due to Earth obscuration of the WXT FoV from 2025-02-05T20:26:42 to 2025-02-05T21:31:28.

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). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.

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