GCN Circular 42548
Subject
EP251102a: Einstein Probe detection of a fast X-ray transient
Event
Date
2025-11-03T12:39:07Z (2 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. F. Liang, H. Zhou (PMOC) and H. W. Pan (NAOC) on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP251102a. The source did not trigger the WXT on-board trigger unit. A GCN Notice was sent manually earlier (tigger ID: 11900459650). The ground analysis of the WXT data shows that the event started at T0=2025-11-02T21:22:22 (UTC) and lasted for around 400 seconds. The WXT position of the source is R.A. = 113.140 deg, DEC = 7.322 deg (J2000) with an uncertainty of 2.4 arcmin in radius (90% C.L. statistical and systematic).
The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 9.1 x 10^20 cm^-2 and a photon index of 1.6(-/+0.1). The derived average unabsorbed 0.5-4 keV flux is 7.2(-1.6/+1.5) x 10^(-10) erg/s/cm^2. The peak flux is around 1.7 x 10^(-9) erg/s/cm^2.
We performed a target-of-opportunity follow-up observation with the Follow-up X-ray Telescope (FXT). Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 113.1642 deg, DEC = 7.3199 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further information will be updated when the FXT telemetry data are received.
We note that an optical counterpart has been reported (GCN 42544, 42546), which is positionally consistent with the FXT source.
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).