GCN 44034: EP260316a: refined analysis of the EP-WXT and EP-FXT observations

S. Q. Jiang (NAO, CAS), B. Zhang, J. P. Feng (USTC), W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:

The fast X-ray transient EP260316a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Jiang et al., GCN 44027). The transient was detected immediately when WXT observation started at T0=2026-03-16 12:32:52. The WXT observation lasted for approximately 22 seconds, and was interrupted due to the autonomous follow-up observation. The WXT light curve exhibits a double-peaked structure, which is also observed in the Fermi/GBM data (Ravasio et al., GCN 44029). The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a hydrogen column density of 2.4 (-1.1/+1.4) × 10^22 cm^-2 and a photon index of 2.9 (-1.4/+1.6). The derived average unabsorbed 0.5-4 keV flux is 3.1 (-2.1/+9.1) × 10^(-8) erg/s/cm^2.

The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-03-16 12:35:40 (UTC, T0+168 s). The exposure time of this observation is 5.4 ks. On-ground analysis of the FXT data found an uncatalogued fading source at R.A., Dec. = 226.9819, 27.4455 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a hydrogen column density of 7.01 (-0.55/+0.57) × 10^21 cm^-2 and a photon index of 2.39 (-0.10/+0.11). The derived average unabsorbed 0.5-10 keV flux is 9.51 (-0.41/+0.47) × 10^(-12) erg/s/cm^2.

A follow-up observation with the EP/FXT was planned, and further information will be updated when the telemetry data are received.

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).