TITLE: GCN CIRCULAR NUMBER: 14588 SUBJECT: GRB 130504C: further Swift-XRT observations DATE: 13/05/06 14:14:28 GMT FROM: Paolo D'Avanzo at INAF-OAB P. D'Avanzo, A. Melandri (INAF-OAB), P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: We have analysed 7.7 ks of XRT data for the Fermi-LAT-detected burst: GRB 130504C, from 65.6 ks to 93.3 ks after the Fermi-LAT trigger. The data are entirely in Photon Counting (PC) mode. The X-ray source reported by D'Avanzo et al. (GCN Circ. 14584) displays behaviour consistent with fading. We propose it as the X-ray afterglow of GRB 130504C. The refined XRT position is RA, Dec = 91.63038, +3.8339 which is equivalent to: RA (J2000): 06 06 31.29 Dec(J2000): +03 50 02.0 with an uncertainty of 3.6 arcsec (radius, 90% confidence). The light curve can be modelled with a power-law decay with a decay index of alpha=1.3 (+/-1.0). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+/-0.3). The best-fitting absorption column is 5.1 (+2.0, -1.6) x 10^21 cm^-2, in excess of the Galactic value of 2.5 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.0 x 10^-11 (7.9 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 5.1 (+2.0, -1.6) x 10^21 cm^-2 Galactic foreground: 2.5 x 10^21 cm^-2 Excess significance: 2.7 sigma Photon index: 1.8 (+/-0.3) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020267. This circular is an official product of the Swift-XRT team. [GCN OPS NOTE(06may13): Per author's request, the extra "further" was removed from the ubject-line.]