{
  "bibcode": "2011GCN.12224....1L",
  "body": "O. M. Littlejohns, P. A. Evans, A. P. Beardmore (U. Leicester) and S.\nR. Oates (UCL-MSSL) report on behalf of the Swift-XRT team:\n\nWe have analysed 8.1 ks of XRT data for GRB 110731A (Oates  et al. GCN\nCirc. 12215), from 56 s to 36.0 ks after the  BAT trigger. The data\ncomprise 607 s in Windowed Timing (WT) mode (the first 8 s were taken\nwhile Swift was slewing) with the remainder in Photon Counting (PC)\nmode. The enhanced XRT position for this burst was given by Beardmore\net al. (GCN. Circ 12219).\n\nThe light curve can be modelled with a series of power-law decays. The\ninitial decay index is alpha=1.04 (+0.05, -0.10). At T+502 s  the decay\nsteepens to an alpha of 1.183 (+0.027, -0.024) before breaking again at\nT+29.0 ks to a final decay with index alpha=4.2 (+5.8, -2.0). We note\nthat the uncertainty in this final decay is large due to the small\nnumber of data points after the final break.\n\nA spectrum formed from the WT mode data can be fitted with an absorbed\npower-law with a photon spectral index\tof 2.08 (+/-0.05). The\nbest-fitting absorption column is  2.05 (+0.14, -0.13) x 10^21 cm^-2,\nin excess of the Galactic value of 1.0 x 10^21 cm^-2 (Kalberla et al.\n2005). The PC mode spectrum has a photon index of 1.92 (+/-0.12) and a\nbest-fitting absorption column of 1.7 (+0.4, -0.3) x 10^21 cm^-2. The\ncounts to observed (unabsorbed) 0.3-10 keV flux conversion factor\ndeduced from this spectrum  is 4.0 x 10^-11 (5.5 x 10^-11) erg cm^-2\ncount^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     1.7 (+0.4, -0.3) x 10^21 cm^-2\nGalactic foreground: 1.0 x 10^21 cm^-2\nExcess significance: 8.4 sigma\nPhoton index:\t     1.92 (+/-0.12)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00458448.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 12224,
  "createdOn": 1312203272000,
  "email": "oml2@le.ac.uk",
  "subject": "GRB 110731A: Swift-XRT refined Analysis",
  "submitter": "Owen Littlejohns at U of Leicester  <oml2@le.ac.uk>",
  "eventId": "GRB 110731A"
}