TITLE: GCN CIRCULAR NUMBER: 40874 SUBJECT: GRB 250628B: EP-FXT counterpart detection DATE: 25/06/30 06:04:47 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn
W. J. Zhang, X. Mao (NAO, CAS), Y. C. Fu (BNU), H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 250628B (SVOM/sb25062804, Dornic et al. GCN #40862) at 2025-06-28T18:51:59 (UTC), about 140 minutes after the SVOM/ECLAIRs trigger, with an exposure time of ~5000s. Five uncatalogued sources are detected within the ECLAIRs error circle. Preliminary analysis of these sources has been conducted automatically, and the details are listed below.
Source 1: EPF_J221230.7-453117 RA (J2000): 333.1278 Dec (J2000): -45.5213 Flux: 6.57 x 10^-14 erg/s/cm2 (FXT-A observed, 0.5-10 kev) Flux_err: 2.48 x 10^-14 erg/s/cm2 (1 sigma)
Source 2: EPF_J221214.3-453344 RA (J2000): 333.0595 Dec (J2000): -45.5621 Flux: 2.13 x 10^-13 erg/s/cm2 (FXT-A observed, 0.5-10 kev) Flux_err: 4.30 x 10^-14 erg/s/cm2 (1 sigma)
Source 3: EPF_J221327.1-453729 RA (J2000): 333.3628 Dec (J2000): -45.6247 Flux: 1.89 x 10^-13 erg/s/cm2 (FXT-A observed, 0.5-10 kev) Flux_err: 4.06 x 10^-14 erg/s/cm2 (1 sigma)
Source 4: EPF_J221218.2-453223 RA (J2000): 333.0758 Dec (J2000): -45.5397 Flux: 9.71 x 10^-14 erg/s/cm2 (FXT-A observed, 0.5-10 kev) Flux_err: 2.74 x 10^-14 erg/s/cm2 (1 sigma)
Source 5: EPF_J221226.9-453909 RA (J2000): 333.1120 Dec (J2000): -45.6525 Flux: 4.23 x 10^-14 erg/s/cm2 (FXT-B observed, 0.5-10 kev) Flux_err: 1.54 x 10^-14 erg/s/cm2 (1 sigma)
Note: Sources 1-3 were simultaneously detected by both FXT-A and FXT-B, while Source 4 and Source 5 were individually detected by FXT-A and FXT-B, respectively.
The position uncertainties of the sources are 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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