TITLE: GCN CIRCULAR NUMBER: 15474 SUBJECT: GRB 131108A: Swift-XRT observations DATE: 13/11/09 15:26:33 GMT FROM: Michael Stroh at PSU/Swift M. C. Stroh and J. A. Kennea (PSU) report on behalf of the Swift-XRT team: We have analysed 7.9 ks of XRT data for the Fermi/LAT-detected burst: GRB 131108A, from 25.9 ks to 49.8 ks after the Fermi/LAT trigger. The data are entirely in Photon Counting (PC) mode. An X-ray source is detected within the Fermi/LAT error circle. Using 4560 s of PC mode data and 6 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 = 156.50178, +9.66248 which is equivalent to: RA (J2000): 10h 26m 00.43s Dec(J2000): +09d 39' 44.9" with an uncertainty of 1.6 arcsec (radius, 90% confidence). This position is 14.4 arcmin from the center of the Fermi/LAT error circle (GCN #15464), consistant with that position, and inside the IPN error box (GCN #15466). The light curve can be modelled with a power-law decay with a decay index of alpha=1.9 (+0.7, -0.6). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.2 (+0.4, -0.3). The best-fitting absorption column is 9.9 (+7.6, -6.5) x 10^20 cm^-2, in excess of the Galactic value of 3.2 x 10^20 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.2 x 10^-11 (4.5 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 9.9 (+7.6, -6.5) x 10^20 cm^-2 Galactic foreground: 3.2 x 10^20 cm^-2 Excess significance: 1.7 sigma Photon index: 2.2 (+0.4, -0.3) If the light curve continues to decay with a power-law decay index of 1.9, the count rate at T+24 hours will be 5.2 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x 10^-13 (2.4 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/00020326. This circular is an official product of the Swift-XRT team.