TITLE: GCN CIRCULAR NUMBER: 26559 SUBJECT: GRB 191221B: Swift-XRT refined Analysis DATE: 19/12/22 17:57:08 GMT FROM: Phil Evans at U of Leicester P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), J. D. Gropp (PSU), A. Tohuvavohu (U. Toronto), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) and S Laha report on behalf of the Swift-XRT team: We have analysed 7.8 ks of XRT data for GRB 191221B (S Laha et al. GCN Circ. 26534), from 74 s to 58.6 ks after the BAT trigger. The data comprise 1.3 ks in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 26540). The late-time light curve (from T0+4.9 ks) can be modelled with a power-law decay with a decay index of alpha=1.03 (+0.09, -0.08). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.097 (+/-0.014). The best-fitting absorption column is consistent with the Galactic value of 8.6 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.86 (+0.07, -0.06) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (4.1 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Galactic foreground: 8.6 x 10^20 cm^-2 Intrinsic column: 8.6 (+5.8, -0.0) x 10^20 cm^-2 at z=1.148 Photon index: 1.86 (+0.07, -0.06) If the light curve continues to decay with a power-law decay index of 1.03, the count rate at T+24 hours will be 0.087 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.1 x 10^-12 (3.6 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00945521. This circular is an official product of the Swift-XRT team.