GRB 241209B: EP-FXT afterglow detection

We performed a follow-up observation of GRB 241209B (SVOM/ECLAIRs and GRM, Xie et al., GCN 38478) with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The observation started at 2024-12-09T16:01:19 (T-TGRB ~ 12.1hr) for about 3ks of exposure in total and a second epoch was also performed at 2024-12-12T22:35:16 (T-TGRB ~ 3.8 days) for again about 3ks of exposure in total. In the first epoch, an uncatalogued X-ray source is detected by both FXT-A and FXT-B at the position (J2000) RA, DEC = 194.6388, 76.1747 (error=10", 90% C.L.), 5.75 arcminute away from the SVOM/ECLAIRs position (Xie et al., GCN 38478). This position is also consistent with the optical afterglow detected by SVOM/VT (Qiu et al., GCN 38516, GCN 38568) and Swift/XRT source 1 (Williams et al., GCN 38525). This source is no longer detected in the second epoch 3.8 days post GRB trigger time confirming it has significantly faded and is indeed the x-ray afterglow of GRB 241209B. 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.