TITLE: GCN CIRCULAR NUMBER: 20828 SUBJECT: GRB 170306A: Swift-XRT refined Analysis DATE: 17/03/06 21:36:26 GMT FROM: Phil Evans at U of Leicester P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and report on behalf of the Swift-XRT team: report on behalf of the Swift-XRT team: We have analysed 6.7 ks of XRT data for GRB 170306A, from 119 s to 28.8 ks after the BAT trigger. The data comprise 29 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 = 263.0690, -44.7482 which is equivalent to: RA (J2000): 17 32 16.56 Dec(J2000): -44 44 53.5 with an uncertainty of 3.6 arcsec (radius, 90% confidence). The light curve can be modelled with an initial power-law decay with an index of alpha=3.25 (+0.28, -0.21), followed by a break at T+559 s to an alpha of 0.36 (+0.10, -0.26). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.05 (+0.30, -0.19). The best-fitting absorption column is 3.98 (+1.64, -0.16) x 10^21 cm^-2, consistent with the Galactic value of 3.8 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 3.8 x 10^-11 (6.1 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.98 (+1.64, -0.16) x 10^21 cm^-2 Galactic foreground: 3.8 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 2.05 (+0.30, -0.19) If the light curve continues to decay with a power-law decay index of 0.36, the count rate at T+24 hours will be 0.011 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.3 x 10^-13 (6.9 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/00741220. This circular is an official product of the Swift-XRT team.