TITLE: GCN CIRCULAR NUMBER: 37316 SUBJECT: GRB 240821A: the second and third EP-FXT follow-up observations and afterglow candidates DATE: 24/08/28 10:49:36 GMT FROM: SVOM_group svomgroup@bao.ac.cn
D. Turpin (CEA), H. Q. Cheng, H. Sun, W. Xie (NAOC, CAS), Y. F. Liang (PMO, CAS), J. Q. Peng (IHEP, CAS), W. Yuan (NAOC, CAS), S. Guillot (IRAP), J. Guan, C. K. Li, Y. Chen, S. M. Jia, W. W. Cui, D. W. Han, W. Li, C. Z. Liu, F. J. Lu, L. M. Song, J. Wang, J. J. Xu, J. Zhang, S. N. Zhang, H. S. Zhao, X. F. Zhao (IHEP, CAS), Y. Liu, C. C. Jin, C. Zhang, Z. X. Ling, J. Wang, L. P. Xin (NAOC, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA) on behalf of the SVOM and Einstein Probe teams
We performed two additional epochs of follow-up observation of GRB 240821A (Cangemi et al, GCN 37220; He et al. GCN 37226; Fermi/GBM GCN 37219) with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The observation started at epoch 2: 2024-08-22T09:46:59 and epoch3 2024-08-26T10:00:08 (T-TGRB ~ 0.63 and 4.64d, respectively ) each for about 8 ks of exposure in total. In addition, we reprocessed the data of the first epoch (Turpin et al., GCN 37230) with 5 ks of total exposure.
In total, considering the 3 epochs of observation, we detected 25 uncatalogued individual X-ray sources within a radius of 13 arcmin centered at RA, DEC = 354.23 deg, -10.18 deg (Cangemi et al, GCN 37220). - 9 sources have only one detection in either epoch 2 or 3 and are not consistent with the Swift/XRT source positions. We do not consider these sources as credible afterglow candidates. - 14 sources are detected in at least two epochs but are consistent, within errors, with no flux variability as function of time. 3 of them are consistent in position and flux measurement with the Swift/XRT source #1, #2 and #4 (Evans et al., GCN 37249). We do not consider these sources as credible afterglow candidates. - 2 sources are detected in the epochs 1 and 2, but were not detected in the third epoch (even if an excess is marginally detected for one of them), showing a clear sign of fading behavior across the three epochs. Therefore, they are considered as credible afterglow candidates.
The following table lists the detailed information of these two X-ray candidates detected by EP-FXT. The positions are given with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The fluxes are given based on the observations of FXT-B. The measured fluxes of the epoch 1 and epoch 2 are calculated in the 0.5 - 10 keV energy range and the uncertainties are at the 90% confidence level. The upper limits of the epoch 3 are calculated using the best-fitting parameter obtained by fitting the X-ray spectra of epoch 2 and given in the 0.5 - 10 keV at the 90% confidence level.
EP-FXT source #1 RA, DEC (J2000) = 354.2701 deg, -10.1911 deg ----------------------------------------------------------- | epoch # | TGRB-Tstart | exposure | Estimated Flux | | | hr | ks | (erg/s/cm^2) | ----------------------------------------------------------- | 1 | 8.8 | 5.0 | 1.1(+/-0.2) x 10^-13 | | 2 | 15.2 | 8.0 | 5.9(+/-1.3) x 10^-14 | | 3 | 111.4 | 8.4 | < 1.5 x 10^-14 | -----------------------------------------------------------
EP-FXT source #2 RA, DEC(J2000) = 354.1516 deg, -10.0504 deg ----------------------------------------------------------- | epoch # | TGRB-Tstart | exposure | Estimated Flux | | | hr | ks | (erg/s/cm^2) | ----------------------------------------------------------- | 1 | 8.8 | 5.0 | 6.0(+/-1.3) x 10^-14 | | 2 | 15.2 | 8.0 | 3.5(+/-1.0) x 10^-14 | | 3 | 111.4 | 8.4 | < 3.4 x 10^-14 | -----------------------------------------------------------
The EP-FXT source #1 is spatially consistent with the Swift/XRT #3 source (Evans et al., GCN 37249). It is the brightest X-ray counterpart found in the first epoch and the decreasing of the flux is compatible with the reported XRT flux within errors.
The EP-FXT source #2 is actually marginally detected in the third epoch thus the upper limit presented in the above table is not significantly lower than the flux measured in the second epoch.
We suggest that the EP-FXT source #1 is the most promising X-ray afterglow candidate of GRB 240821A. We strongly encourage further optical and NIR follow-up of this sky position RA, DEC (J2000) = 354.2701 deg, -10.1911 deg (10” error radius) with large telescopes to potentially detect late time kilonova/supernova emission associated with this short GRB+extended emission.
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.
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