GCN Circular 45150
Subject
GRB 260708A: slowly decaying X-ray emission observed by EP/FXT
Event
Date
2026-07-14T02:48:42Z (8 days ago)
From
Rui-Zhi Li at Yunnan Observatories, CAS <liruizhi@ynao.ac.cn>
Via
Web form
R.-Z. Li (YNAO, CAS), J. Mao (YNAO, CAS), S. Antier (IJCLAB), M. Pillas (IAP), D. Turpin (CEA/Irfu), O. Pyshna (Caltech) report on behalf of a larger collaboration:
We report two EP/FXT Target of Opportunity (ToO) observations of the Fermi long GRB 260708A (Fermi/GBM Team, GCN 45115; Fermi/LAT, Holzmann Airasca et al., GCN 45117; GECAM-B, Ren et al., GCN 45126; Glowbug, Cheung et al., GCN 45130).
The first EP/FXT follow-up observation started at 2026-07-11T01:06:38 (UTC), approximately 2.62 days after the Fermi/GBM trigger, with a total exposure time of 2.7 ks.
EP/FXT detected an uncatalogued X-ray source at the following coordinates (J2000):
R.A., Decl. = 225.6537, -43.8844
with a localization uncertainty of 10 arcsec radius (90% confidence level, including both statistical and systematic uncertainties). The X-ray position is consistent with that of AT 2026stf reported by MASTER (Shilova et al., GCN 45123).
The average 0.5-10 keV EP/FXT spectrum can be fitted by an absorbed power law with a Galactic hydrogen column density of (0.36 +/- 0.12) x 10^22 cm^-2 and a photon index of 2.25 +/- 0.32. The derived average unabsorbed 0.5-10 keV flux is (1.31 +/- 0.13) x 10^-12 erg cm^-2 s^-1.
The second EP/FXT follow-up observation started at 2026-07-13T12:09:39 (UTC), approximately 5.08 days after the Fermi/GBM trigger, with a total exposure time of 6 ks. On-ground analysis of the EP/FXT data again detected an uncatalogued X-ray source consistent with the source detected in the first observation. The average 0.5-10 keV EP/FXT spectrum can be fitted by an absorbed power law with a Galactic hydrogen column density of (0.40 +/- 0.5) x 10^22 cm^-2 and a photon index of 2.36 +/- 0.38. The derived average unabsorbed 0.5-10 keV flux is (5.46 +/- 0.68) x 10^-13 erg cm^-2 s^-1.
The temporal decay index of X-ray emission between the two observations is 1.32 +/- 0.24. Given the slow temporal evolution observed in both the optical band (Shilova et al., GCN 45123; Freeberg et al., GCN 45127; Shi et al., GCN 45140; Sheng et al., GCN 45147) and the X-ray band, optical spectroscopy is strongly encouraged to determine the classification of the source.
All quoted uncertainties correspond to the 1-sigma confidence level.
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).