{
  "bibcode": "2017GCN.22174....1O",
  "body": "J.P. Osborne (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri\n(INAF-OAB), P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), A. Tohuvavohu\n(PSU), S. J. LaPorte (PSU), K.L. Page (U. Leicester), A.P. Beardmore\n(U. Leicester) and P.A. Evans (U. Leicester) report on behalf of the\nSwift-XRT team:\n\nSwift-XRT has conducted further observations of the field of the\nFermi/LAT-detected burst GRB 171124A (Longo et al. GCN Circ. 22156).\nThe observations now extend from T0+21.8 ks to T0+579.1 ks. \n\nOf the sources reported by Osborne et al. (GCN Circ. 22165), \"Source 2\"\nis fading with 3-sigma significance, and is therefore likely the GRB\nafterglow. Using 4982 s of PC mode data and 3 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 = 335.47598, +35.33213\nwhich is equivalent to:\n\nRA (J2000): 22h 21m 54.24s\nDec(J2000): +35d 19' 55.7\"\n\nwith an uncertainty of 2.3 arcsec (radius, 90% confidence). This\nposition is 7.1 arcmin from the Fermi/LAT position.  The source is\nfading with alpha >0.7.\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.8 (+0.8, -0.7). The\nbest-fitting absorption column is  7.9 (+4.7, -3.5) x 10^21 cm^-2, in\nexcess of the Galactic value of 1.3 x 10^21 cm^-2 (Willingale et al.\n2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 2.9 x 10^-11 (1.0 x 10^-10) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     7.9 (+4.7, -3.5) x 10^21 cm^-2\nGalactic foreground: 1.3 x 10^21 cm^-2\nExcess significance: 3.1 sigma\nPhoton index:\t     2.8 (+0.8, -0.7)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00020784.\nThe results of the full analysis of the XRT observations are available\nat http://www.swift.ac.uk/ToO_GRBs/00020784.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 22174,
  "createdOn": 1512127268000,
  "email": "pae9@leicester.ac.uk",
  "subject": "GRB 171124A: Swift-XRT afterglow detection",
  "submitter": "Phil Evans at U of Leicester  <pae9@leicester.ac.uk>",
  "eventId": "GRB 171124A"
}