GCN Circular 42749

S. Q. Jiang (NAO, CAS), Q. C. Liu (THU), D. F. Hu, R. Shi, Q. J. Huang (PMO, CAS), F. F. Song (YNAO, CAS), C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:

We report on the detection of a fast X-ray transient EP251118a by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient did not trigger the WXT onboard unit. A GCN Notice was sent manually (trigger ID: 11900480769). The WXT light curve started at 2025-11-18T16:42:29 (UTC).

The position of the source is R.A. = 123.160 deg, DEC = 10.952 deg (J2000) with an uncertainty of 1.87 arcmin (radius, 90% C.L. statistical and systematic). The transient event has a precursor-like signature lasted about 42 seconds followed by a main burst lasting at least 79 seconds. After that, the observation ended, resulting in no further data.

The average 0.5-4.0 keV spectrum can be fitted with an absorbed powerlaw model, with free nH of 2.26 (-0.26/+0.27) x 10^21 cm^-2 and a photon index of 1.32 (-0.08/+0.08). The unabsorbed average 0.5-4.0 keV flux is estimated to be 2.50 (-0.08/+0.08) x 10^-8 erg/s/cm2. The peak flux reaches around 1 x 10^-7 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters.

Further FXT observations are 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).