GCN Circular 43215
R.Shi, Q. J. Huang (PMO, CAS), C. L. Guo, X. Mao, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP251222b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Guo et al., GCN 43201), which is associated with GRB 251222A (Yang et al., GCN 43188), and followed by several telescopes (Palmerio et al., GCN 43189, Evans et al., GCN 43191, An et al., GCN 43194, Li et al., GCN 43203, Saccardi et al., GCN 43204, Gupta et al., GCN 43207, Saccardi et al., GCN 43208, Kumar et al., GCN 43209, Pérez-Fournon et al., GCN 43212, Beardmore et al., GCN 43213). The refined analysis of the WXT data shows that the event started at T0=2025-12-22T17:04:52 (UTC) and lasted for ~350 s with the peak flux of 1.4e-8 erg/s/cm^2. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 9.81e20 cm^-2 and a photon index of 1.1 (-/+0.15). The derived average unabsorbed 0.5-4 keV flux is 2.8 (-0.2/+0.3) x 10^(-9) erg/s/cm^2.
The observation by the Follow-up X-ray Telescope (FXT) was performed at 2025-12-23T05:11:10 (UTC), about 12.11 hours after T0. The exposure time of this observation is 1884 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 77.1954, -7.2139 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 fixed Galactic equivalent hydrogen column density of 9.81e20 cm^-2 and a photon index of 1.86 (-0.25/+0.26). The derived average unabsorbed 0.5-10 keV flux is 1.8 (-0.3/+0.4) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
Further observations are encouraged.
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).