TITLE:   GCN CIRCULAR
NUMBER:  39664
SUBJECT: EP250311a/GRB 250311A: EP-WXT detection
DATE:    25/03/11 10:28:02 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

X. Mao (NAO, CAS), Y. J. Zhang (THU), Y. L. Hua (PMO, CAS), Z. X. Ling (NAO, CAS) report on behalf of the Einstein Probe team:

We report on the detection of a fast X-ray transient EP250311a at 2025-03-11T01:56:19 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The position of the source is R.A. = 224.691 deg, DEC = -2.850 deg (J2000) with an uncertainty of 2.8 arcmin (radius, 90% C.L. statistical and systematic). EP250311a may be associated with GRB 250311A/MAXI J1457-026 reported by the MAXI team (GCN #39660) due to their temporal and spatial coincidence.

The transient event lasted for about 150 seconds, with a peak flux of about 1.6 x 10^-9 erg/s/cm^2 in 0.5-4 keV. The average spectrum during the burst can be fitted with an absorbed powerlaw model, with nH fixed at the Galactic value of 6.4 x 10^20 cm^-2 and a photon index of 1.4 (-0.8/+0.8). The unabsorbed average 0.5-4 keV flux is estimated to be 6.1 (-1.9/+3.1) x 10^-10 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters.

The FXT follow-up observations have been scheduled. Further follow-up observations are encouraged to identify the nature of this X-ray transient.

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).