TITLE: GCN CIRCULAR NUMBER: 15623 SUBJECT: GRB 131227A: Swift-XRT refined Analysis DATE: 13/12/27 17:09:04 GMT FROM: Phil Evans at U of Leicester V. D'Elia (ASDC), A. Maselli (INAF-IASFPA), M.C. Stroh (PSU), D.N. Burrows (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB) and S.R. Oates report on behalf of the Swift-XRT team: We have analysed 8.6 ks of XRT data for GRB 131227A (Oates et al. GCN Circ. 15616), from 63 s to 18.8 ks after the BAT trigger. The data comprise 1.8 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. Using 4789 s of PC mode data and 7 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 = 67.37831, +28.88296 which is equivalent to: RA (J2000): 04h 29m 30.80s Dec(J2000): +28d 52' 58.7" with an uncertainty of 1.4 arcsec (radius, 90% confidence). The light curve can be modelled with a power-law decay with a decay index of alpha=1.110 (+0.029, -0.028). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.28 (+0.22, -0.21). The best-fitting absorption column is 4.4 (+0.9, -0.8) x 10^21 cm^-2, in excess of the Galactic value of 1.5 x 10^21 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 1.93 (+0.22, -0.21) and a best-fitting absorption column of 4.9 (+1.2, -1.1) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.9 x 10^-11 (8.1 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 4.9 (+1.2, -1.1) x 10^21 cm^-2 Galactic foreground: 1.5 x 10^21 cm^-2 Excess significance: 5.3 sigma Photon index: 1.93 (+0.22, -0.21) If the light curve continues to decay with a power-law decay index of 1.110, the count rate at T+24 hours will be 3.6 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x 10^-13 (2.9 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/00582184. This circular is an official product of the Swift-XRT team.