TITLE: GCN CIRCULAR NUMBER: 16848 SUBJECT: GRB 140928A: Swift-XRT observations DATE: 14/09/29 08:35:47 GMT FROM: Claudio Pagani at U of Leicester C. Pagani, K. L. Page and P. A. Evans (U. Leicester) report on behalf of the Swift-XRT team: We have analysed 2.5 ks of XRT data for the Fermi/LAT-detected burst: GRB 140928A (Desiante et al., GCN Circ. 16847) from 40.7 ks to 52.7 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 2472 s of PC mode data and 2 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 = 43.69878, -55.92883 which is equivalent to: RA (J2000): 02h 54m 47.71s Dec(J2000): -55d 55' 43.8" with an uncertainty of 1.5 arcsec (radius, 90% confidence). This position is 588 arcsec from the Fermi/LAT position. The light curve is consistent with a constant source of mean count rate 1.9e-01 ct/sec. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.83 (+0.26, -0.24). The best-fitting absorption column is 2.4 (+1.0, -0.9) x 10^21 cm^-2, in excess of the Galactic value of 1.6 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 3.3 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.4 (+1.0, -0.9) x 10^21 cm^-2 Galactic foreground: 1.6 x 10^20 cm^-2 Excess significance: 4.2 sigma Photon index: 1.83 (+0.26, -0.24) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020418. This circular is an official product of the Swift-XRT team.