TITLE: GCN CIRCULAR NUMBER: 23125 SUBJECT: GRB 180809B: Swift-XRT refined Analysis DATE: 18/08/10 20:40:03 GMT FROM: Phil Evans at U of Leicester Z. Liu (NAOC / U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and M.J. Moss report on behalf of the Swift-XRT team: We have analysed 3.9 ks of XRT data for GRB 180809B (Moss et al. GCN Circ. 23105), from 74 s to 12.6 ks after the BAT trigger. The data comprise 1.7 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. Using 1724 s of PC mode data and 4 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 299.69997, -15.29893 which is equivalent to: RA (J2000): 19h 58m 47.99s Dec(J2000): -15d 17' 56.1" with an uncertainty of 1.4 arcsec (radius, 90% confidence). The late-time light curve (from T0+4.7 ks) can be modelled with a power-law decay with a decay index of alpha=1.37 (+/-0.12). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.609 (+/-0.017). The best-fitting absorption column is 2.29 (+0.09, -0.08) x 10^21 cm^-2, in excess of the Galactic value of 1.2 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.63 (+/-0.11) and a best-fitting absorption column of 2.4 (+/-0.5) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.6 x 10^-11 (5.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.4 (+/-0.5) x 10^21 cm^-2 Galactic foreground: 1.2 x 10^21 cm^-2 Excess significance: 4.0 sigma Photon index: 1.63 (+/-0.11) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00852553. This circular is an official product of the Swift-XRT team.