TITLE: GCN CIRCULAR NUMBER: 31072 SUBJECT: GRB 211107B: Swift-XRT afterglow detection DATE: 21/11/10 11:36:34 GMT FROM: Phil Evans at U of Leicester <pae9@leicester.ac.uk> A. D'Ai (INAF-IASFPA), D.N. Burrows (PSU), J. D. Gropp (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A. Melandri (INAF-OAB), T. Sbarrato (INAF-OAB) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has conducted further observations of the field of the Swift/BAT-detected burst GRB 211107B (Swift/BAT did not successfully trigger on GRB 211107B et al. GCN Circ. 31057). The observations now extend from T0+124.3 ks to T0+147.5 ks. The source previously reported by D'Ai et al. (GCN Circ. 31064), "Source 1", is fading with 1.8 sigma significance and thus is believed to be the GRB afterglow. Using 2958 s of PC mode data and 4 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 = 65.80812, -27.56084 which is equivalent to: RA (J2000): 04h 23m 13.95s Dec(J2000): -27d 33' 39.0" with an uncertainty of 2.7 arcsec (radius, 90% confidence). This position is 51 arcsec from the Swift/BAT position. The light curve can be modelled with a power-law decay with a decay index of alpha=2.84 (+0.04, -2.67). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.5 (+0.9, -0.7). The best-fitting absorption column is 1.6 (+2.5, -1.2) x 10^21 cm^-2, consistent with the Galactic value of 4.3 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 2.7 x 10^-11 (4.5 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.6 (+2.5, -1.2) x 10^21 cm^-2 Galactic foreground: 4.3 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 2.5 (+0.9, -0.7) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021467. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021467. This circular is an official product of the Swift-XRT team.