TITLE: GCN CIRCULAR NUMBER: 17517 SUBJECT: GRB 150301A: Swift-XRT refined Analysis DATE: 15/03/01 20:45:59 GMT FROM: Phil Evans at U of Leicester M. de Pasquale (INAF-IASFPA), A. Maselli (INAF-IASFPA), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), P.A. Evans (U. Leicester), A. Amaral-Rogers (U. Leicester), J.P. Osborne (U. Leicester), A. Melandri (INAF-OAB) and A.Y. Lien report on behalf of the Swift-XRT team: We have analysed 147 s of XRT data for GRB 150301A (Lien et al. GCN Circ. 17510), from 50 s to 43.7 ks after the BAT trigger. The data are entirely in Windowed Timing (WT) mode (the first 6 s were taken while Swift was slewing). The light curve can be modelled with an initial power-law decay with an index of alpha=0.53 (+0.18, -0.59), followed by a break at T+112 s to an alpha of 2.6 (+0.6, -0.7). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.71 (+0.21, -0.20). The best-fitting absorption column is 5.6 (+/-0.7) x 10^22 cm^-2, in excess of the Galactic value of 1.3 x 10^22 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 9.1 x 10^-11 (2.0 x 10^-10) erg cm^-2 count^-1. A summary of the WT-mode spectrum is thus: Total column: 5.6 (+/-0.7) x 10^22 cm^-2 Galactic foreground: 1.3 x 10^22 cm^-2 Excess significance: 10.5 sigma Photon index: 1.71 (+0.21, -0.20) If the light curve continues to decay with a power-law decay index of 2.6, the count rate at T+24 hours will be 9.7 x 10^-7 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.8 x 10^-17 (1.9 x 10^-16) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00632995. This circular is an official product of the Swift-XRT team.