GCN Circular 38606
Subject
EP241217b/GRB 241217A: EP detection of the prompt X-ray emssion
Date
2024-12-18T06:20:25Z (3 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
EP241217b/GRB 241217A: EP detection of the prompt X-ray emssion
H. Zhou (PMO, CAS), S.-F. Zhu (USTC), M. H. Zhang (NAO, CAS), C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:
We report on the detection of an X-ray transient by the Einstein Probe (EP), designated as EP241217b (Trigger ID: 01709129080).
About 10 seconds after the trigger of GRB 241217A (GCN 38594), the Einstein Probe (EP) began to observe the scheduled field which happened to cover GRB 241217A. EP241217b triggered the on-board processing unit of the Wide-field X-ray Telescope (WXT) at 2024-12-17T16:58:04.349 (UTC), i.e., about 53 seconds after the trigger of GRB 241217A. The WXT position of the X-ray source is R.A. = 84.167 deg, Dec. = -25.281 deg, with an uncertainty of 2.8 arcmin (90%).
About 134 seconds after the trigger of the GRB, the Follow-up X-ray Telescope (FXT) onborad EP performed an autonomous follow-up observation, and found an uncatalogued X-ray source, located at R.A., Dec. = 84.1499, -25.3003 deg with an uncertainty of about 20 arcsec (90%). The FXT position is consistent with the X-ray/optical afterglow positions of GRB 241217A (GCN 38594, 38599, 38600).
EP241217b was detected by the WXT at the begining of the scheduled observation, i.e., 10 seconds after the GRB trigger. The WXT light curve (including the follow-up observation) clearly shows 2 pulses seperated in time. The first one lasts from 10 to 620s, and it is composed of two short pulses peaked at 150s and 260s, respectively (from 53 to 134 seconds, EP was slewing, the possiblity there could be other pulses in this period can not be ruled out). The second one lasts from 1000 to 1500s. The similar temporal behavior is also seen in the FXT light curve.
The time averaged WXT spectrum (about 134 to 2000 seconds after the trigger of the GRB) was fitted by an absorbed powerlaw model with the fixed Milky Way equivalent hydrogen column density of 1.79 x 10^20 cm^-2 and an intrinsic absorber assuming the redshift of the absorper is 1.195 (the Legacy Survey dr9 photo-z of the extended source overlayed with EP241217b/GRB 241217A). The best fitted model shows the equivalent hydrogen column density of the intrinsic absorption is 1.42(-0.46, +0.51) x 10^22 cm^-2, the spectral index is 1.57(-0.21, +0.22) and the unabsorbed 0.5-4 keV flux is (1.19+/-0.10) x 10^-9 erg/s/cm^2.
The FXT-A spectrum (about 134 to 7800 seconds after the triger of the GRB) is extracted with an annular region to mitigate the potential pile-up effect. The FXT spectrum is then fitted using the same model as the WXT spectrum. The best-fit intrinsic nH is 0.85(+/-0.06) x 10 ^ 22 cm^-2, with a power-law photon index of 1.58(+/-0.04). The unabsorbed 0.5-10 keV flux is measured at 1.1 x 10 ^-9 erg/s/cm^2.
A further FXT follow-up observation has been arranged.
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.