{
  "bibcode": "2016GCN.20316....1B",
  "body": "A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), P. D'Avanzo\n(INAF-OAB), A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU), B. Sbarufatti\n(INAF-OAB/PSU), D.N. Burrows (PSU), S.L. Gibson (U. Leicester) and V.\nD'Elia report on behalf of the Swift-XRT team:\n\nWe have analysed 3.4 ks of XRT data for GRB 161220A (D'Elia et al. GCN\nCirc. 20302), from 60 s to 58.3 ks after the  BAT trigger. The data\ncomprise 85 s in Windowed Timing (WT) mode (the first 7 s were taken\nwhile Swift was slewing) with the remainder in Photon Counting (PC)\nmode. The refined XRT position is RA, Dec = 350.8479, -47.4978 which is\nequivalent to:\n\nRA (J2000): 23 23 23.49\nDec(J2000): -47 29 52.2\n\nwith an uncertainty of 3.6 arcsec (radius, 90% confidence).\n\nThe light curve can be modelled with a series of power-law decays. The\nlight curve initially rises, with an index alpha=-1.5 (+0.0, -2.0). At\nT+71.0 s it breaks to an alpha of 4.75 (+0.51, -0.29) before breaking\nagain at T+217 s to a final decay with index alpha=0.82 (+0.13, -0.11).\n\nA spectrum formed from the WT mode data can be fitted with an absorbed\npower-law with a photon spectral index\tof 3.57 (+0.20, -0.19). The\nbest-fitting absorption column is  3.2 (+/-0.4) x 10^21 cm^-2, in\nexcess of the Galactic value of 1.2 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 2.1 x 10^-11 (9.9 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the WT-mode spectrum is thus:\nTotal column:\t     3.2 (+/-0.4) x 10^21 cm^-2\nGalactic foreground: 1.2 x 10^20 cm^-2\nExcess significance: 12.4 sigma\nPhoton index:\t     3.57 (+0.20, -0.19)\n\nIf the light curve continues to decay with a power-law decay index of\n0.82, the count rate at T+24 hours will be 2.7 x 10^-3 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.7 x\n10^-14 (2.6 x 10^-13) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00727610.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 20316,
  "createdOn": 1482310564000,
  "email": "pae9@leicester.ac.uk",
  "subject": "GRB 161220A: Swift-XRT refined Analysis",
  "submitter": "Phil Evans at U of Leicester  <pae9@leicester.ac.uk>",
  "eventId": "GRB 161220A"
}