TITLE:   GCN CIRCULAR
NUMBER:  41361
SUBJECT: GRB 250813B: EP-FXT counterpart detection
DATE:    25/08/14 09:01:39 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

Q. Y. Wu, H. Q. Cheng, Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 250813B (SVOM/sb25081303, Xie et al. GCN #41352, #41354) at 2025-08-14 00:58:26(UTC), about 2 hours after the SVOM/ECLAIRs trigger, with an exposure time of ~1.3 ks. One uncatalogued source is detected within the ECLAIRs error circle, and the source is spatially consistent with the counterpart reported in optical and X-ray bands (Xie et al. GCN #41352, #41354, Jelinek et al. GCN #41353, #41355, Gill et al. GCN #41356, Page et al. GCN #41359). Preliminary analysis on this source are conducted (note that the exposure time for the analysed data is not complete), with the details listed as follows. 

Source 1: EPF_J222700.2+122741
RA (J2000): 336.7507
Dec (J2000): 12.4617
Flux: 1.00 x 10^-11 erg/s/cm2 (observed, 0.5-10 kev)
Flux_err: 1.2 x 10^-12 erg/s/cm2 (90% C. L.)

The position uncertainty of the source is about 10 arcsec in radius (90% C.L. statistical and systematic).

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).