TITLE: GCN CIRCULAR NUMBER: 30914 SUBJECT: GRB 210928A: Swift-XRT refined Analysis DATE: 21/10/01 17:29:32 GMT FROM: Phil Evans at U of Leicester <pae9@leicester.ac.uk> V. D'Elia (SSDC & INAF-OAR), A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), A. Melandri (INAF-OAB) and report on behalf of the Swift-XRT team: report on behalf of the Swift-XRT team: We have analysed 5.3 ks of XRT data for the -detected burst GRB 210928A, from 67.4 ks to 262.9 ks after the trigger. The data are entirely in Photon Counting (PC) mode. Using 1636 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 = 245.51109, -17.94788 which is equivalent to: RA (J2000): 16h 22m 02.66s Dec(J2000): -17d 56' 52.4" with an uncertainty of 4.4 arcsec (radius, 90% confidence). This position is 6793.5 arcmin from the position. The light curve can be modelled with a power-law decay with a decay index of alpha=1.3 (+1.8, -1.3). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.4 (+0.6, -0.3). The best-fitting absorption column is 2.342 (+3.358, -0.021) x 10^21 cm^-2, consistent with the Galactic value of 2.3 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 4.8 x 10^-11 (5.5 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.342 (+3.358, -0.021) x 10^21 cm^-2 Galactic foreground: 2.3 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.4 (+0.6, -0.3) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00014850. This circular is an official product of the Swift-XRT team.