TITLE: GCN CIRCULAR NUMBER: 41467 SUBJECT: EP250821a: Preliminary analysis of the EP-WXT and EP-FXT observations DATE: 25/08/21 16:27:06 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

Y. F. Liang, D. F. Hu (PMO, CAS), C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:

The X-ray transient EP250821a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 41459). We report on the further results of WXT and FXT observations. The analysis of the WXT data shows that the event started at T0=2025-08-21T07:17:04 (UTC) and lasted for at least 80 s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 7.33 x 10^20 cm^-2 and a photon index of 1.2 (-/+0.5). The derived average unabsorbed 0.5-4 keV flux is 1.3(-/+0.4) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 320 s after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 289.8862, DEC = -43.3808 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), within the WXT error circle. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.25 x 10^21 cm^-2 and a photon index around 3.5. The derived average unabsorbed 0.5-10 keV flux is about 2.6 x 10^(-10) erg/s/cm^2 during the time interval from 0 to 1000 seconds after the start of the observation.The results from FXT may have been affected by pile-up effect, particularly in the brightest time interval. Therefore, the values reported here should be considered preliminary and are intended to provide a rough estimate of the source intensity.

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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