TITLE:   GCN CIRCULAR
NUMBER:  43682
SUBJECT: GRB 260208B: refined analysis of the EP-FXT observation
DATE:    26/02/10 12:51:13 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

S. Q. Jiang, H. N. Yang, Y. L. Wang, Y. Liu (NAO, CAS), Z. H. Yang, Q. C. Zhao (IHEP, CAS) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation on GRB 260208B (The Fermi GBM team, GCN 43638; Preis & Greiner, GCN 43639; Longo et al., GCN 43647; Sonawane et al., GCN 43649; Luo et al., GCN 43650; Harsha et al., GCN 43677) at 2026-02-09T06:37:35 (UTC), about 20.748 hours after the Fermi/GBM trigger.
With a 4.6 ks exposure, the afterglow (Li et al., GCN 43659; Magnani et al., GCN 43668; Beardmore et al., GCN 43671) is clearly detected by EP-FXT.

The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 4.3 × 10^21 cm^-2, and a photon index of 2.01 (-0.21/+0.21). The derived average unabsorbed 0.5-10 keV flux is 7.28 (-0.81/+0.97) × 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

Compared to the flux reported by Swift/XRT (Beardmore et al., GCN 43671), the temporal decay index is approximately -1.4.

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).