TITLE: GCN CIRCULAR NUMBER: 41592 SUBJECT: EP250828a: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/08/29 13:13:17 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn
K. R. Ni (CCNU), H. L. Peng (NNU), H. Y. Liu, Y. J. Song and H. Sun (NAO,CAS) on behalf of the Einstein Probe (EP) team:
We report on the refined analysis of EP-WXT and FXT observations of the fast X-ray transient EP250828a (Liu et al., GCN 41574), with the optical couterparts observed by Seimei (Taguchi et al., GCN 41582), SVOM/VT (Yao et al., GCN 41583), NOT (Fu et al., GCN 41584), Kinder (Aryan et al., GCN 41591). The WXT data shows that the event started at T0=2025-08-28T07:27:16 (UTC) and lasted for around 1500s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 7.75 x 10^20 cm^-2 and a photon index of 1.48 (-0.59/+0.63). The derived average unabsorbed 0.5-4 keV flux is 1.25(-0.39/+0.50) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
The EP-FXT observed the field of EP250828a about 1.4 hrs after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 327.8932, DEC = 20.5429 (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 7.75 x 10^20 cm^-2 and a photon index of 2.04(-0.41/+0.43). The derived average unabsorbed 0.5-10 keV flux is 4.43 (-1.29/+1.90) x 10^(-13)erg/s/cm^2. 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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