TITLE:   GCN CIRCULAR
NUMBER:  45625
SUBJECT: GRB 260917B: EP-FXT follow-up observation
DATE:    26/09/18 10:20:25 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

Q. J. Huang (PMO, CAS), K. R. Ni (CCNU), T. Wu (AHNU), H. W. Pan (NAO, CAS)  on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of GRB 260917B, detected by SVOM/ECLAIRs (sb26091701; He et al., GCN 45606), which was also observed in optical by SVOM/COLIBRÍ (Dornic et al., GCN 45607), SVOM/VT (Ma et al., GCN 45610), LCO ( Yin et al., GCN 45608), WINTER (Mohan et al., GCN 45611) and FRAM-Auger (Jelinek et al., GCN 45617). The observation started at 2026-09-17 07:43:08 (UTC), about 3.85 hours after the SVOM/ECLAIRs trigger, with an exposure time of 2985 s.

On-ground analysis of the EP-FXT data found six uncataloged X-ray sources within the SVOM/ECLAIRs error region (He et al., GCN 45606). The position of Source 6 is consistent with that of the optical counterpart candidate (Dornic et al., GCN 45607), and its X-ray flux shows a declining trend. The preliminary results for the X-ray sources are as follows:

Source 1: EPF_J030731.7+252560
R.A., Decl. (J2000) = 46.8822, 25.4332 deg
Localization error radius = 10 arcsec
0.5-10 keV flux = 7.56e-14 erg/cm^ 2/s
Flux error = 2.67e-14 erg/cm^2/s (1 sigma)
Separation = 3.17 arcmin

Source 2: EPF_J030721.6+252434
R.A., Decl. (J2000) = 46.8399, 25.4095 deg
Localization error radius = 10 arcsec
0.5-10 keV flux = 5.91e-14 erg/cm^ 2/s
Flux error = 2.39e-14 erg/cm^2/s (1 sigma)
Separation = 4.64 arcmin

Source 3: EPF_J030705.6+252807
R.A., Decl. (J2000) = 46.7734, 25.4687 deg
Localization error radius = 10 arcsec
0.5-10 keV flux = 5.03e-14 erg/cm^ 2/s
Flux error = 1.58e-14 erg/cm^2/s (1 sigma)
Separation = 5.02 arcmin

Source 4: EPF_J030738.5+252413
R.A., Decl. (J2000) = 46.9105, 25.4035 deg
Localization error radius = 10 arcsec
0.5-10 keV flux = 1.97e-13 erg/cm^ 2/s
Flux error = 4.29e-14 erg/cm^2/s (1 sigma)
Separation = 5.41 arcmin

Source 5: EPF_J030758.4+253204
R.A., Decl. (J2000) = 46.9934, 25.5343 deg
Localization error radius = 10 arcsec
0.5-10 keV flux = 8.03e-14 erg/cm^ 2/s
Flux error =  2.21e-14erg/cm^2/s (1 sigma)
Separation = 7.61 arcmin

Source 6: EPF_J030755.8+253327
R.A., Decl. (J2000) = 46.9823, 25.5576 deg
Localization error radius = 10 arcsec
0.5-10 keV flux = 6.96e-13 erg/cm^ 2/s
Flux error = 1.83e-13 erg/cm^2/s (1 sigma)
Separation = 7.77 arcmin

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).