TITLE: GCN CIRCULAR NUMBER: 20962 SUBJECT: GRB 170331A: Swift-XRT refined Analysis DATE: 17/03/31 17:20:27 GMT FROM: Phil Evans at U of Leicester S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A. Cholden-Brown (PSU) and report on behalf of the Swift-XRT team: report on behalf of the Swift-XRT team: We have analysed 7.4 ks of XRT data for GRB 170331A, from 127 s to 19.2 ks after the BAT trigger. The data comprise 12 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The light curve can be modelled with a power-law decay with a decay index of alpha=0.99 (+0.12, -0.10). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.5 (+/-0.3). The best-fitting absorption column is 5.6 (+1.7, -1.5) x 10^21 cm^-2, in excess of the Galactic value of 4.7 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 3.3 x 10^-11 (7.8 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 5.6 (+1.7, -1.5) x 10^21 cm^-2 Galactic foreground: 4.7 x 10^20 cm^-2 Excess significance: 5.8 sigma Photon index: 2.5 (+/-0.3) If the light curve continues to decay with a power-law decay index of 0.99, the count rate at T+24 hours will be 1.1 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.7 x 10^-14 (8.9 x 10^-14) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00744791. This circular is an official product of the Swift-XRT team.