{
  "bibcode": "2016GCN.19585....1M",
  "body": "A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), D.N.\nBurrows (PSU), T.G.R. Roegiers (PSU), L.M. McCauley (PSU), S.L. Gibson\n(U. Leicester), J.P. Osborne (U. Leicester) and P.A. Evans (U.\nLeicester) report on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the\nFermi/LAT-detected burst GRB 160625B (Dirirsa et al. GCN Circ. 19580)\nin a series of observations tiled on the sky. The total exposure time\nis 2.7 ks, distributed over 8 tiles; the maximum exposure at a single\nsky location was 1.1 ks. The data were collected between T0+9.6 ks and\nT0+10.0 ks, and are entirely in Photon Counting (PC) mode. \n\nAn uncatalogued X-ray source is detected and is above the RASS limit,\nand is therefore likely the GRB afterglow.  The position of this source\nis RA, Dec=308.5969, +6.9196 which is equivalent to:\n\nRA (J2000): 20:34:23.25\nDec(J2000): +06:55:10.5\n\nwith an uncertainty of 3.8 arcsec (radius, 90% confidence).  This\nposition is 19.7 arcmin from the Fermi/LAT position. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=3.156 (+0.011, -2.764).\n\nA spectrum formed from the WT mode data can be fitted with an absorbed\npower-law with a photon spectral index\tof 2.09 (+0.20, -0.19). The\nbest-fitting absorption column is  1.6 (+/-0.6) x 10^21 cm^-2, in\nexcess of the Galactic value of 9.8 x 10^20 cm^-2 (Willingale et al.\n2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 3.1 x 10^-11 (4.3 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the WT-mode spectrum is thus:\nTotal column:\t     1.6 (+/-0.6) x 10^21 cm^-2\nGalactic foreground: 9.8 x 10^20 cm^-2\nExcess significance: 1.8 sigma\nPhoton index:\t     2.09 (+0.20, -0.19)\n\nIf the light curve continues to decay with a power-law decay index of\n3.156, the count rate at T+24 hours will be 6.1 x 10^-3 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.9 x\n10^-13 (2.6 x 10^-13) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis of the likely afterglow\nare at http://www.swift.ac.uk/xrt_products/TILED_GRB00056/index_1.php.\nThe results of the full analysis of the tiled XRT observations are\navailable at http://www.swift.ac.uk/xrt_products/TILED_GRB00056.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 19585,
  "createdOn": 1466924053000,
  "email": "antonino.dai@ifc.inaf.it",
  "subject": "GRB 160625B: Swift-XRT afterglow detection",
  "submitter": "Antonino D'Ai at IASF-PA  <antonino.dai@ifc.inaf.it>",
  "eventId": "GRB 160625B"
}