{
  "bibcode": "2011GCN.12364....1M",
  "body": "V. Mangano (INAF-IASFPA), B. Sbarufatti (INAF-OAB/IASFPA), P. A. Evans\n(U Leicester), H. A. Krimm (GSFC/USRA)\treport on behalf of the\nSwift-XRT team:\n\nWe have analysed 2.5 ks of XRT data for the IPN-detected burst: GRB\n110918A (Hurley et al. et al. GCN Circ. 12357), from 107.4 ks to 113.6\nks after the  IPN trigger. The data are entirely in Photon Counting\n(PC) mode.  An new X-ray source is detected close to the edge of the\nXRT FoV. Using 2498 s of PC mode data and 6 UVOT images, we find an\nenhanced XRT position (using the XRT-UVOT alignment and matching UVOT\nfield sources to the USNO-B1 catalogue): RA, Dec = 32.53860, -27.10610\nwhich is equivalent to:\n\nRA (J2000): 02h 10m 9.27s\nDec(J2000): -27d 06' 22.0\"\n\nwith an uncertainty of 1.5 arcsec (radius, 90% confidence). This\nposition is 640 arcsec from the IPN position, and outside the IPN error\nbox.  The light curve uses only the first 1.5 ks of data, since after\nthis the source is too near the edge of the field of view for the light\ncurve extraction tools to work. This light curve is consistent with a\nconstant source of mean count rate 2.7e-01 ct/sec. A power-law fit\ngives an index of -1.320 (+3.525, -0.013). The source count rate is\ndefinitely above the 3 sigma upper limit of 0.08 ct/sec obtained from\nthe analysis of RASS data.\nWe believe that such a bright uncatalogued X-ray source close to the\nIPN error box is likely the afterglow of the GRB.\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.12 (+0.28, -0.23). The\nbest-fitting absorption column is  1.5 (+0.7, -0.6) x 10^21 cm^-2, in\nexcess of the Galactic value of 1.7 x 10^20 cm^-2 (Kalberla et al.\n2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 2.9 x 10^-11 (4.2 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     1.5 (+0.7, -0.6) x 10^21 cm^-2\nGalactic foreground: 1.7 x 10^20 cm^-2\nExcess significance: 3.9 sigma\nPhoton index:\t     2.12 (+0.28, -0.23)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00020186.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 12364,
  "createdOn": 1316525556000,
  "email": "vanessa@ifc.inaf.it",
  "subject": "GRB 110918A: Swift-XRT observations",
  "submitter": "Vanessa Mangano at INAF-IASFPA  <vanessa@ifc.inaf.it>",
  "eventId": "GRB 110918A"
}