GCN Circular 40956
Subject
EP250704a: refined analysis of the EP-WXT and FXT follow-up observations
Event
Date
2025-07-04T16:47:42Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
A. Li (BNU), Y.H Cheng (SWIFAR,YNU), C. Zhou (HUST), G.Y. Zhao (SYSU), Y. J. Zhang (THU) and W. D. Zhang (NAOC) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP250704a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (GCN 40941), and followed by Swift XRT (GCN 40951) and several optical telescopes (GCN 40942, GCN 40945, GCN 40947). The refined analysis of the WXT data shows that the event started at 2025-07-04T08:16:27 (UTC) and lasted for 10 s with the peak flux of 6 x 10^(-7) erg/s/cm^2, 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 8.0 x 10^20 cm^-2 and a photon index of 1.7 (-/+1.3). The derived average unabsorbed 0.5-4 keV flux is 1.3 (-1.1/+0.8) x 10^(-9) erg/s/cm^2 over the 10 seconds of the pulse.
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2025-07-04 08:18:59 (UTC), about two minutes later. The exposure time of this observation is 495 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 300.8721, 12.0230 deg (J2000) with an uncertainty of about 10 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 Galactic equivalent hydrogen column density of 8.0 x 10^20 cm^-2 and a photon index of 2.05 (-/+0.07). The derived average unabsorbed 0.5-10 keV flux is 9.0 (-/+0.5) x 10^(-11) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
Further follow-up observations with EP-FXT have been scheduled.
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).