TITLE: GCN CIRCULAR NUMBER: 28205 SUBJECT: GRB 200803A: Swift-XRT afterglow detection DATE: 20/08/03 20:28:04 GMT FROM: Andy Beardmore at U Leicester J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D. Gropp (PSU) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Fermi/LAT-detected burst GRB 200803A (Cutini et al. GCN Circ. 28203) in a series of observations tiled on the sky. The total exposure time is 3.5 ks, distributed over 4 tiles; the maximum exposure at a single sky location was 1.6 ks. The data were collected between T0+36.3 ks and T0+46.6 ks, and are entirely in Photon Counting (PC) mode. Five uncatalogued X-ray sources are detected, of which one ("Source 1") is above the RASS limit, and is therefore likely the GRB afterglow. Using 812 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 = 311.09390, -62.94478 which is equivalent to: RA (J2000): 20h 44m 22.54s Dec(J2000): -62d 56' 41.2" with an uncertainty of 2.8 arcsec (radius, 90% confidence). This position is 9.7 arcmin from the Fermi/LAT position and 1.5 arcsec from the MASTER optical counterpart reported by Lipunov et al. (GCN Circ. 28202). The light curve can be modelled with a power-law decay with a decay index of alpha=3.30 (+0.06, -2.56). If the light curve continues to decay with a power-law decay index of 3.30, the count rate at T+24 hours will be 2.1 x 10^-3 count s^-1 The results of the XRT-team automatic analysis of the likely afterglow are at https://www.swift.ac.uk/xrt_products/TILED_GRB00092/Source1.php. The results of the full analysis of the tiled XRT observations are available at https://www.swift.ac.uk/xrt_products/TILED_GRB00092. This circular is an official product of the Swift-XRT team.