EP251222b
GCN Circular 43215
R.Shi, Q. J. Huang (PMO, CAS), C. L. Guo, X. Mao, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP251222b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Guo et al., GCN 43201), which is associated with GRB 251222A (Yang et al., GCN 43188), and followed by several telescopes (Palmerio et al., GCN 43189, Evans et al., GCN 43191, An et al., GCN 43194, Li et al., GCN 43203, Saccardi et al., GCN 43204, Gupta et al., GCN 43207, Saccardi et al., GCN 43208, Kumar et al., GCN 43209, Pérez-Fournon et al., GCN 43212, Beardmore et al., GCN 43213). The refined analysis of the WXT data shows that the event started at T0=2025-12-22T17:04:52 (UTC) and lasted for ~350 s with the peak flux of 1.4e-8 erg/s/cm^2. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 9.81e20 cm^-2 and a photon index of 1.1 (-/+0.15). The derived average unabsorbed 0.5-4 keV flux is 2.8 (-0.2/+0.3) x 10^(-9) erg/s/cm^2.
The observation by the Follow-up X-ray Telescope (FXT) was performed at 2025-12-23T05:11:10 (UTC), about 12.11 hours after T0. The exposure time of this observation is 1884 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 77.1954, -7.2139 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 9.81e20 cm^-2 and a photon index of 1.86 (-0.25/+0.26). The derived average unabsorbed 0.5-10 keV flux is 1.8 (-0.3/+0.4) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
Further observations are encouraged.
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).
GCN Circular 43202
SVOM/GRM team: Chen-Wei Wang, Wen-Jun Tan, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)
SVOM/ECLAIRs team: Hui Yang, Sebastien Guillot(IRAP)
Report on behalf of the SVOM team:
SVOM/GRM was triggered in-flight by a burst GRB 251222A (SVOM trigger reference: sb25122207) at 2025-12-22T17:05:52.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN#43186), SVOM/ECLAIRs (Yang et al., GCN#43188) and EP (Guo et al., GCN#43201).
With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multiple pulses with a T90 of 85.5 +3.0/-3.5 s in the 15-5000 keV band.
The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb251222A.png
In addition, the position of this burst, as determined by SVOM/ECLAIRs (RA= 77.202 deg, DEC= -7.206 deg, ERR=29 arcsec), is located at about 34 degrees from the SVOM optical axis.
With this localization, the time-averaged spectrum from T0-20 to T0+100 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.49 +0.12/-0.12 and the cutoff energy, parameterized as Epeak, is 221 +233/-60 keV. The event fluence (10-1000 keV) in this time interval is (1.37 +0.17/-0.14)E-05 erg/cm^2.
The 1s peak spectrum, measured from T0+53 to T0+54 s, is also best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.95 +0.12/-0.12 and the cutoff energy, parameterized as Epeak, is 209 +40/-28 keV. The flux (10-1000 keV) in this time interval is (1.05 +0.09/-0.08)E-06 erg/cm^2/s.
The localization of GRB 251222A in the 'Amati' relation diagram is shown at:
https://www.bursthub.cn//admin/static/svgrb251222A_amati.png
And the localization in the 'Yonetoku' relation diagram is shown at:
https://www.bursthub.cn//admin/static/svgrb251222A_yonetoku.png
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.
The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang@ihep.ac.cn)
GCN Circular 43201
C.-L. Guo (NAO, CAS), R. Shi (PMO, CAS), Q.-J. Huang (PMO,CAS), X. Mao (NAO, CAS), and Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP251222b. The transient triggered EP-WXT (ID: 01709250454) at 2025-12-22 17:05:34 (UTC). The WXT position of the source is R.A. = 77.207 deg, DEC = -7.208 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). EP251222b is located 0.8 arcmin away from GRB 251222A (GCN 43189), and the trigger time is differ by only 12 seconds. Thus EP251222b is possibly associated with GRB 251222A.
Further information will be updated when the telemetry data are received.
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).