TITLE: GCN CIRCULAR NUMBER: 38905 SUBJECT: EP250111a: a new X-ray transient detected by Einstein Probe DATE: 25/01/11 08:44:14 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

T. Zhao (NAO, CAS), X. L. Chen (YNU), Q. C. Shui (IHEP, CAS), Kaushik Chatterjee (YNU), T. Y. Lian and C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team

We report on the detection of an X-ray transient designated EP250111a by the Einstein Probe (EP) mission, which triggered the on-board processing unit at 2025-01-11T01:20:44 (UTC) (trigger ID: 01709130131). The transient event started at 2025-01-11T01:20:24 (UTC) and lasted for 83s before the observation was interrupted by the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 1.11 x 10^21 cm^-2 and a photon index of 1.01 (-0.52/+0.54). The derived average observed 0.5-4 keV flux is 1.39 (-0.40/+0.53) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

An autonomous observation was performed by the Follow-up X-ray Telescope (FXT) about four minutes later. An uncatalogued source was detected at R.A. = 97.1809, DEC = 56.8983 (J2000) with an uncertainty of about 20 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 1.11 x 10^21 cm^-2 and a photon index of 2.42 (-0.14/+0.15). The derived average observed 0.5-10 keV flux is 3.43 (-0.33/+0.36) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

More information on this source will be updated when the full telemetry data is received. Further follow-up observations are encouraged to identify the nature of this X-ray transient.

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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