TITLE: GCN CIRCULAR NUMBER: 41256 SUBJECT: EP250806a: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/08/07 04:42:31 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

R. D. Liang, H. N. Yang (NAO, CAS), Y. J. Zhang (THU), W. D. Zhang (NAO, CAS) report on behalf of the Einstein Probe team:

The X-ray transient EP250806a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Yang et al., GCN 41246). Refined analysis of the WXT data shows that the event started at T0=2025-08-06 09:02:10.82 (UTC) and lasted for about 50 s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.47 x 10^20 cm^-2 and a photon index of 2.3 (-/+0.6). The derived average unabsorbed 0.5-4 keV flux is 4.1(-/+0.5) x 10^(-11) erg/s/cm^2.

The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 1 ks after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 335.1028, DEC = 26.8738 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent positionally with the WXT transient. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.47 x 10^20 cm^-2 and a photon index of 3.24 (-/+0.25). The derived average unabsorbed 0.5-10 keV flux is 8.42 (-/+0.47) x 10^(-13) erg/s/cm^2 during the time interval from 0 to 1000 seconds after the start of the observation. The uncertainties are at the 90% confidence level for the above parameters.

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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