GCN Circular 38740
Subject
GRB 241229A: EP-FXT X-ray afterglow confirmation
Event
Date
2024-12-31T10:22:37Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
S. Q. Jiang, J. W. Hu, H. Sun, W. J. Zhang, X. P. Xu (NAO, CAS), W. F. Wen (SZTU), J. H. Wu (GU), H.W. Pan, Z. X. Ling (NAO, CAS), B. Cordier (CEA) on behalf of Einstein Probe teams:
We performed further follow-up observations of GRB 241229A (SVOM/ECLAIRs, Liang et al., GCN 38697) with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The first epoch of the observation started at 2024-12-30T09:24:30 (T-TGRB ~ 1.39 days) with an exposure time of 2 ks, and a second epoch was performed at 2024-12-30T16:06:04 (T-TGRB ~ 1.67 days) with an exposure time of 5 ks.
The flux of the first (EPF_J125139.4+315311) and the third (EPF_J125101.6+315007) sources reported in Jiang et al., GCN 38726 remained almost constant over all the observations.
The second source EPF_J125129.3+314453, which is also detected by Swift/XRT (Williams et al., GCN 38728) shows a clear flux decay with a powerlaw index of ~ 0.8. The unabsorbed 0.5-10 keV flux given in the below table is calculated by assuming the same spectral index 1.86 and fixed with the Milky Way equivalent hydrogen column density nH of 1.43 x 10^20 cm^-2:
Observation start time [UTC] | exposure time [s] | 0.5 - 10 keV unabsorbed flux [erg/s/cm2]
2024-12-29T14:11:35 | 2828 | 7.94 (+2.12 -1.53) x 10^(-13)
2024-12-30T09:24:30 | 2212 | 4.19 (+0.81 -0.72) x 10^(-13)
2024-12-30T16:06:04 | 4898 | 3.50 (+1.14 -0.77) x 10^(-13)
Thus we claim that this source is very likely the afterglow of GRB 241229A.
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.