TITLE: GCN CIRCULAR NUMBER: 18109 SUBJECT: GRB 150801B: Swift-XRT refined Analysis DATE: 15/08/02 18:14:27 GMT FROM: Boris Sbarufatti at INAF-OAB/IASFPA P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), T.G.R. Roegiers (PSU), L.M. McCauley (PSU), J.A. Kennea (PSU), B.P. Gompertz (U. Leicester), J.P. Osborne (U. Leicester), A. D'ai (INAF-IASFPA) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift-XRT team: We have analysed 12 ks of XRT data for GRB 150801B (Sbarufatti et al. GCN Circ. 18104), from 64 s to 40.0 ks after the BAT trigger. We have analysed 12 ks of XRT data for GRB 150801B (Sbarufatti et al. GCN Circ. 18104), from 64 s to 40.0 ks after the BAT trigger. The data comprise 8 s in Windowed Timing (WT) mode while Swift was slewing to the burst, 270 s in PC mode, a further 185 s in WT mode during a bright flare starting at T0+366 s (also seen in the BAT), with the remainder 11.5 ks in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 18108). The late-time light curve (from T0+4.9 ks) can be modelled with a power-law decay with a decay index of alpha=1.74 (+0.18, -0.17). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.79 (+0.21, -0.19). The best-fitting absorption column is 3.0 (+/-0.4) x 10^22 cm^-2, in excess of the Galactic value of 2.7 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 7.6 x 10^-11 (1.5 x 10^-10) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.0 (+/-0.4) x 10^22 cm^-2 Galactic foreground: 2.7 x 10^21 cm^-2 Excess significance: 10.8 sigma Photon index: 1.79 (+0.21, -0.19) If the light curve continues to decay with a power-law decay index of 1.74, the count rate at T+24 hours will be 1.4 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x 10^-13 (2.2 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/00650977. This circular is an official product of the Swift-XRT team.