TITLE: GCN CIRCULAR NUMBER: 28740 SUBJECT: GRB 201020B: Swift-XRT afterglow detection DATE: 20/10/22 09:04:39 GMT FROM: Paolo D'Avanzo at INAF - OAB B. Sbarufatti (PSU), D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), 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 201020B (Arimoto et al. GCN Circ. 28716), collecting 1.7 ks of Photon Counting (PC) mode data between T0+72.1 ks and T0+74.3 ks. An uncatalogued X-ray source is detected and is above the RASS limit, and is therefore likely the GRB afterglow. Using 1015 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 = 75.46680, +77.06830 which is equivalent to: RA (J2000): 05h 01m 52.03s Dec(J2000): +77d 04' 05.9" with an uncertainty of 2.0 arcsec (radius, 90% confidence). This position is 9.6 arcmin from the Fermi/LAT position and consistent with the optical counterpart detected by MASTER (Lipunov et al. GCN Circ. 28718). The light curve is consistent with a constant source of mean count rate 1.5e-01 ct/sec. With the currently available dataset we cannot constrain the temporal evolution. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.2 (+0.4, -0.3). The best-fitting absorption column is 4.0 (+1.7, -1.4) x 10^21 cm^-2, in excess of the Galactic value of 1.4 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 3.3 x 10^-11 (5.7 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 4.0 (+1.7, -1.4) x 10^21 cm^-2 Galactic foreground: 1.4 x 10^21 cm^-2 Excess significance: 3.0 sigma Photon index: 2.2 (+0.4, -0.3) The results of the XRT-team automatic analysis of the likely afterglow are at https://www.swift.ac.uk/ToO_GRBs/00021035/Source1.php. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021035. This circular is an official product of the Swift-XRT team.