GCN Circular 41339
Subject
EP250812a: refined analysis of the EP-WXT and EP-FXT observations
Event
Date
2025-08-13T08:57:14Z (5 days ago)
Edited On
2025-08-13T15:03:00Z (5 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
M.H. Zhang (NAO, CAS), H. Zhou (PMO, CAS), W. D. Zhang (NAO, CAS) report on behalf of the Einstein Probe team:
The X-ray transient EP250812a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhang et al., GCN 41327). Refined analysis of the WXT data shows that the event started at T0=2025-08-12 13:09:03 (UTC) and lasted for about 37 s before the interruption of the autonomous follow-up. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 2.5 x 10^20 cm^-2 and a photon index of 1.0 (-0.3/+0.3). The derived average unabsorbed 0.5-4 keV flux is 6.7(-0.8/+0.8) x 10^(-10) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 186 s after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 30.9931, DEC = -9.3191 (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 2.5 x 10^20 cm^-2 and a photon index of 2.21 (-0.18/+0.20). The absorption from the host galaxy cannot be constrained. The derived average unabsorbed 0.5-10 keV flux is 1.97 (-0.13/+0.21) x 10^(-12) 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).