TITLE: GCN CIRCULAR NUMBER: 41424 SUBJECT: GRB 250818B: EP-FXT counterpart detection DATE: 25/08/19 02:25:18 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

R.-Z. Li (YNAO, CAS), Y.-C. Fu (BNU), W.-D. Zhang and Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 250818B (SVOM/sb25081801, Wang et al. GCN 41405) at 2025-08-18 06:00:27 (UTC), about 2.52 hours after the SVOM/ECLAIRs trigger, with an exposure time of ~2 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 (Kumar et al. GCN 41406, Ferro et al. GCN 41407, Yao et al. GCN 41409, Zheng et al. GCN 41417, Fong et al. GCN 41419). Preliminary analysis on this source are conducted, with the details listed as follows.

Source 1: EPF_J030413.4-030730 RA (J2000): 46.056 Dec (J2000): -3.1247 Flux: 7.5432e-12 erg/s/cm2 (observed, 0.5-10 kev) Flux_err: 6.1924e-13 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).

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