TITLE: GCN CIRCULAR NUMBER: 42782 SUBJECT: GRB 251118A: Swift-XRT afterglow detection DATE: 25/11/20 13:53:24 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-OAR), D.N. Burrows (PSU), S. Lanava (PSU), S. Dichiara (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Fermi/LAT-detected burst GRB 251118A. We searched for X-ray sources in 3.0 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 5.4 ks, obtained between T0+31.3 ks and T0+108.8 ks. Two uncatalogued X-ray sources are detected, of which one ("Source 1") is above the RASS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. Using 3939 s of PC mode data and 7 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 160.00418, -34.52259 which is equivalent to: RA (J2000): 10h 40m 01.00s Dec(J2000): -34d 31' 21.3" with an uncertainty of 2.0 arcsec (radius, 90% confidence). This position is 5.0 arcmin from the Fermi/LAT position. The light curve can be modelled with a power-law decay with a decay index of alpha=1.08 (+0.18, -0.16). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.73 (+0.16, -0.15). The best-fitting absorption column is 2.2 (+0.7, -0.6) x 10^21 cm^-2, in excess of the Galactic value of 8.3 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.1 x 10^-11 (5.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.2 (+0.7, -0.6) x 10^21 cm^-2 Galactic foreground: 8.3 x 10^20 cm^-2 Excess significance: 3.9 sigma Photon index: 1.73 (+0.16, -0.15) The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/xrt_products/00021890. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021890. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/42782. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...