TITLE: GCN CIRCULAR NUMBER: 17203 SUBJECT: GRB 141220A: Swift-XRT refined Analysis DATE: 14/12/20 20:23:04 GMT FROM: Phil Evans at U of Leicester B.P. Gompertz (U. Leicester), M. de Pasquale (INAF-IASFPA), A. Maselli (INAF-IASFPA), A. Melandri (INAF-OAB), M.C. Stroh (PSU), D.N. Burrows (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and J.R. Cummings report on behalf of the Swift-XRT team: We have analysed 6.8 ks of XRT data for GRB 141220A (Cummings et al. GCN Circ. 17196), from 87 s to 36.3 ks after the BAT trigger. The data comprise 42 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The refined XRT position is RA, Dec = 195.0653, +32.1466 which is equivalent to: RA (J2000): 13 00 15.66 Dec(J2000): +32 08 47.9 with an uncertainty of 3.6 arcsec (radius, 90% confidence). The late-time light curve (from T0+5.6 ks) can be modelled with a power-law decay with a decay index of alpha=1.38 (+0.26, -0.23). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.81 (+0.21, -0.20). The best-fitting absorption column is 8.3 (+5.9, -5.1) x 10^20 cm^-2, in excess of the Galactic value of 1.3 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 3.7 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 8.3 (+5.9, -5.1) x 10^20 cm^-2 Galactic foreground: 1.3 x 10^20 cm^-2 Excess significance: 2.3 sigma Photon index: 1.81 (+0.21, -0.20) If the light curve continues to decay with a power-law decay index of 1.38, the count rate at T+24 hours will be 2.6 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.5 x 10^-14 (1.1 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00621915. This circular is an official product of the Swift-XRT team.