TITLE: GCN CIRCULAR NUMBER: 42356 SUBJECT: GRB 251016A: Swift-XRT afterglow detection DATE: 25/10/18 13:49:20 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , S. Lanava (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of GRB 251016A. We searched for X-ray sources in 3.5 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 5.6 ks, obtained between T0+6.6 ks and T0+107.3 ks. Thirty-two uncatalogued X-ray sources are detected, of which one ("Source 1") is fading with >3-sigma significance, and is therefore likely the GRB afterglow. Using 1190 s of PC mode data and 2 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 = 266.30279, -39.56008 which is equivalent to: RA (J2000): 17h 45m 12.67s Dec(J2000): -39d 33' 36.3" with an uncertainty of 3.2 arcsec (radius, 90% confidence). The light curve can be modelled with a power-law decay with a decay index of alpha=4.53 (+0.08, -3.08). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.4 (+1.0, -0.6). The best-fitting absorption column is 4.8 (+8.8, -0.9) x 10^21 cm^-2, consistent with the Galactic value of 3.9 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.4 x 10^-11 (8.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 4.8 (+8.8, -0.9) x 10^21 cm^-2 Galactic foreground: 3.9 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.4 (+1.0, -0.6) The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/xrt_products/03000141. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/SVOM/SVOM_FIELD00042. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/42356. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...