GRB 240821A: EP-FXT follow-up observation and afterglow candidates

We performed a follow-up observation of GRB 240821A (detected by SVOM/ECLAIRs Cangemi et al, GCN 37220; SVOM/GRM He et al. GCN 37226 and Fermi/GBM GCN 37219) with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The observation began at 2024-08-22T03:22:05 (T-TGRB ~ 9 hrs), and the exposure time is about 3.0 ks. FXT-A detected 2 uncatalogued sources in the 90% localization error circle provided by SVOM/ECLAIRs (with a radius of 13 arcmin centered at RA, DEC = 354.23 deg, -10.18 deg), and 7 are detected by FXT-B. The following table lists the detailed information of the X-ray sources detected by FXT-A and FXT-B. The positions are given with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The uncertainties of flux are at the 90% confidence level. FXT-A candidate list: Source name | RA | DEC | Estimated Flux | SNR | Dist from SVOM/ECLAIRs | | deg | deg | (erg/s/cm^2) | | offset (in arcmin) | ------------------------------------------------------------------------------------------------- EP J233704.9-101126 | 354.2703 | -10.1905 | 1.3(+/-0.3) x 10^-13 | 7.6 | 2.46 | EP J233641.3-100257 | 354.1720 | -10.0491 | 5.5(+/-2.2) x 10^-14 | 4.2 | 8.57 | ------------------------------------------------------------------------------------------------- FXT-B candidate list: Source name | RA | DEC | Estimated Flux | SNR | Dist from SVOM/ECLAIRs | | deg | deg | (erg/s/cm^2) | | offset (in arcmin) | ------------------------------------------------------------------------------------------------- EP J233704.9-101126 | 354.2696 | -10.1893 | 1.2(+/-0.3) x 10^-13 | 8.5 | 2.40 | EP J233641.6-101036 | 354.1735 | -10.1767 | 7.5(+/-2.2) x 10^-14 | 5.1 | 3.34 | EP J233642.1-101318 | 354.1754 | -10.2217 | 5.9(+/-2.0) x 10^-14 | 3.8 | 4.08 | EP J233708.1-101515 | 354.2836 | -10.2542 | 3.0(+/-1.3) x 10^-14 | 4.3 | 5.46 | EP J233710.3-100324 | 354.2929 | -10.0566 | 8.4(+/-3.0) x 10^-14 | 3.9 | 8.28 | EP J233727.8-100744 | 354.3657 | -10.1290 | 4.0(+/-1.6) x 10^-14 | 4.6 | 8.58 | EP J233636.5-100302 | 354.1522 | -10.0507 | 7.8(+/-2.2) x 10^-14 | 6.3 | 9.02 | ------------------------------------------------------------------------------------------------- The above observation was made with the EP-FXT instrument. 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). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.