TITLE: GCN CIRCULAR NUMBER: 35587 SUBJECT: GRB 240118A: Swift-XRT afterglow detection DATE: 24/01/20 22:41:31 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), A. Tohuvavohu (U. Toronto), D.N. Burrows (PSU), J. D. Gropp (PSU), J.P. Osborne (U. Leicester) 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 240118A, collecting 4.7 ks of Photon Counting (PC) mode data between T0+96.2 ks and T0+109.0 ks. Two uncatalogued X-ray sources are detected consistent with being within 296 arcsec of the Fermi/LAT position, of which one ("Source 3") is believed to be the afterglow. Using 4697 s of PC mode data and 3 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 = 18.53587, +29.79798 which is equivalent to: RA (J2000): 01h 14m 08.61s Dec(J2000): +29d 47' 52.7" with an uncertainty of 2.7 arcsec (radius, 90% confidence). This position is 2.1 arcmin from the Fermi/LAT position. The light curve is consistent with a constant source of mean count rate 3.6e-02 ct/sec. A power-law fit gives an index of 1.9 (+1.0, -2.8). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.2 (+/-0.6). The best-fitting absorption column is 1.1 (+0.8, -0.6) x 10^22 cm^-2, in excess of the Galactic value of 7.1 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 4.5 x 10^-11 (9.9 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.1 (+0.8, -0.6) x 10^22 cm^-2 Galactic foreground: 7.1 x 10^20 cm^-2 Excess significance: 3.1 sigma Photon index: 2.2 (+/-0.6) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021644. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021644. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/35587. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...