TITLE: GCN CIRCULAR NUMBER: 43600 SUBJECT: GRB 260131A/B: EP/FXT X-ray afterglow detection DATE: 26/02/02 08:14:51 GMT FROM: sqjiang at NAOC <sqjiang@bao.ac.cn> S.Q. Jiang, D. Xu (NAOC), B.P. Gompertz (U. Birmingham), H.W. Pan (NAOC), K.R. Ni (CCNU), C.Y. Dai (NJU), B.T. Wang (YNAO) report: EP/FXT performed a follow-up observation on GRB 260131A/B detected by Fermi (Fermi/GBM team, GCN 43579; Roberts et al., GCN 43585), MAXI (Sugai et al., GCN 43580), Konus-Wind (Frederiks et al., GCN 43590), and Insight-HXMT (Luo et al., GCN 43591) at 2026-02-01T16:45:27 (UTC), about 34.1 hrs after the Fermi/GBM trigger. An uncatalogued source is detected at R.A. = 24.7137, Dec. = 34.3284 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the optical counterpart GOTO26akj/AT2026bwg (Gompertz et al., GCN 43586). The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 4.81 × 10^20 cm^-2, an intrinsic hydrogen column density upper limit of 4.66 × 10^21 cm^-2 at the redshift of 1.434 (Saccardi et al., GCN 43595), and a photon index of 2.01 (-0.09/+0.12). The derived average unabsorbed 0.5-10 keV flux is 3.35 (-0.19/+0.23) × 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. View this GCN Circular online at https://gcn.nasa.gov/circulars/43600. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...