TITLE: GCN CIRCULAR NUMBER: 18329 SUBJECT: GRB 150915A: Swift-XRT refined Analysis DATE: 15/09/17 13:20:26 GMT FROM: Valerio D'Elia at ASDC A. D'ai (INAF-IASFPA), A. Maselli (INAF-IASFPA), A. Melandri (INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), L. Izzo (URoma/ICRA) and V. D'Elia (ASDC) report on behalf of the Swift-XRT team: We have analysed 12 ks of XRT data for the Swift/BAT-detected burst GRB 150915A (D'Elia et al. GCN Circ. 18315), from 118 s to 126.0 ks after the Swift/BAT trigger. The data comprise 256 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The refined XRT position is RA, Dec = 319.6584, -34.9136 which is equivalent to: RA (J2000): 21 18 38.01 Dec(J2000): -34 54 49.1 with an uncertainty of 3.7 arcsec (radius, 90% confidence). This position is 5.1 arcmin from the Swift/BAT position. The light curve can be modelled with an initial power-law decay with an index of alpha=0.5 (+/-0.3), followed by a break at T+170 s to an alpha of 3.94 (+0.15, -0.13). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.79 (+/-0.04). The best-fitting absorption column is 7.8 (+1.7, -1.6) x 10^21 cm^-2, at a redshift of 1.968, in addition to the Galactic value of 8.6 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 3.7 x 10^-11 (4.5 x 10^-11) erg cm^-2 count^-1. A summary of the WT-mode spectrum is thus: Galactic foreground: 8.6 x 10^20 cm^-2 Intrinsic column: 7.8 (+1.7, -1.6) x 10^21 cm^-2 at z=1.968 Photon index: 1.79 (+/-0.04) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00655721. This circular is an official product of the Swift-XRT team.