TITLE: GCN CIRCULAR NUMBER: 14502 SUBJECT: GRB 130427A: Improved Swift-XRT analysis DATE: 13/04/29 14:32:17 GMT FROM: Phil Evans at U of Leicester P.A. Evans, K.L. Page (U. Leicester), A. Maselli, V. Mangano, M. Capalbi (INAF-IASFPA), D.N. Burrows (PSU) report on behalf of the Swift-XRT team: We have reanalysed the XRT data for GRB 130427A (Maselli et al. GCN Circ. 14448) from 144 s to 141 ks after the BAT trigger. The data comprise 1.9 ks of Windowed Timing (WT) mode data, with the rest taken in Photon Counting (PC) mode. The star trackers on Swift failed to find a correct aspect solution during the first 1.9 ks of exposure (when the WT mode data were collected; see GCN Circ. 14448), and the GRB is seen to be drifting on the XRT detector. We have fitted this drift as a linear function, which is a good representation to the data. We have then rebuilt the GRB light curve and spectrum using this information. However, the time-tag of individual photon events may be incorrect by up to 1 second, and the method that we have used to correct the attitude may introduce low-level fluctuations. It is therefore inappropriate to perform any detailed timing or periodicity analysis with the current XRT data. We are working to rectify this situation. The light curve can be modelled as a broken power-law with an initial decay index of 2.81 (+/- 0.04) with a break at T+421 s to a slope of alpha=1.17 (+/-0.01). This then breaks again at T+53.4ks to a final slope of 1.79 (+0.5, -0.05). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.91 (+/-0.11). The best fitting absorption column is 2.8 (+/-0.05) x 10^21 cm^-2 at a redshift of 0.34, in addition to the Galactic value of 1.8 x 10^20 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (5.2 x 10^-11) erg cm^-2 count^-1. If the light curve continues to decay with a power-law decay index of 1.79, the count rate at T+3 days will be 0.14 count/sec, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.3 x 10^-12 (7.3 x 10^-12) erg cm^-2 s^-1. The automated light curve of this GRB is online at http://www.swift.ac.uk/xrt_curves/00554620/ This circular is an official product of the Swift-XRT team.