GCN Circular 43216
Subject
EP251222a: refined analysis of the EP-WXT and EP-FXT observations
Event
Date
2025-12-23T14:23:50Z (2 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
C.-L. Guo (NAO, CAS), R. Shi, Q.-J. Huang (PMO,CAS), X. Mao, and Y. Liu (NAO,
CAS) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP251222a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Huang et al., GCN 43200), and followed by several telescopes (Li et al., GCN 43205, Kumar et al., GCN 43210, Malesani et al., GCN 43211).
The refined analysis of the WXT data shows that this observation of the event started at T0=2025-12-22T14:59:14 (UTC). During the observation period of WXT (1100s), there was no significant short-timescale variability in the light curve. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 7.23 x 10^20 cm^-2 and a photon index of 1.58 (-0.4/+0.4). The derived average unabsorbed 0.5-4 keV flux is 6.35 (-4.9/+8.2) x 10^(-11) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2025-12-22T15:21:49 (UTC), about 22 minutes after T0. The exposure time of this observation is 4081s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 62.5014, DEC = -1.4795 (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 7.23 x 10^20 cm^-2 and a photon index of 2.34 (-0.05/+0.05). The derived average unabsorbed 0.5-10 keV flux is 7.54 (-5.2/+16.7) x 10^(-12) erg/s/cm^2.
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).