TITLE: GCN CIRCULAR NUMBER: 12364 SUBJECT: GRB 110918A: Swift-XRT observations DATE: 11/09/20 13:32:36 GMT FROM: Vanessa Mangano at INAF-IASFPA V. Mangano (INAF-IASFPA), B. Sbarufatti (INAF-OAB/IASFPA), P. A. Evans (U Leicester), H. A. Krimm (GSFC/USRA) report on behalf of the Swift-XRT team: We have analysed 2.5 ks of XRT data for the IPN-detected burst: GRB 110918A (Hurley et al. et al. GCN Circ. 12357), from 107.4 ks to 113.6 ks after the IPN trigger. The data are entirely in Photon Counting (PC) mode. An new X-ray source is detected close to the edge of the XRT FoV. Using 2498 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 = 32.53860, -27.10610 which is equivalent to: RA (J2000): 02h 10m 9.27s Dec(J2000): -27d 06' 22.0" with an uncertainty of 1.5 arcsec (radius, 90% confidence). This position is 640 arcsec from the IPN position, and outside the IPN error box. The light curve uses only the first 1.5 ks of data, since after this the source is too near the edge of the field of view for the light curve extraction tools to work. This light curve is consistent with a constant source of mean count rate 2.7e-01 ct/sec. A power-law fit gives an index of -1.320 (+3.525, -0.013). The source count rate is definitely above the 3 sigma upper limit of 0.08 ct/sec obtained from the analysis of RASS data. We believe that such a bright uncatalogued X-ray source close to the IPN error box is likely the afterglow of the GRB. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.12 (+0.28, -0.23). The best-fitting absorption column is 1.5 (+0.7, -0.6) x 10^21 cm^-2, in excess of the Galactic value of 1.7 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 2.9 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.5 (+0.7, -0.6) x 10^21 cm^-2 Galactic foreground: 1.7 x 10^20 cm^-2 Excess significance: 3.9 sigma Photon index: 2.12 (+0.28, -0.23) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020186. This circular is an official product of the Swift-XRT team.