{
  "bibcode": "2017GCN.21182....1D",
  "body": "P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti\n(INAF-OAB/PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), B. Mingo\n(U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia (ASDC), A.\nD'Ai (INAF-IASFPA) and P.A. Evans report on behalf of the Swift-XRT\nteam:\n\nWe have analysed 6.8 ks of XRT data for GRB 170531B (Evans et al. GCN\nCirc. 21171), from 129 s to 52.1 ks after the  BAT trigger. The data\ncomprise 303 s in Windowed Timing (WT) mode (the first 10 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 Osborne et\nal. (GCN Circ. 21175).\n\nThe late-time light curve (from T0+5.5 ks) can be modelled with a\npower-law decay with a decay index of alpha=1.18 (+0.25, -0.20).\n\nA spectrum formed from the WT mode data can be fitted with an absorbed\npower-law with a photon spectral index\tof 1.45 (+/-0.04). The\nbest-fitting absorption column is  6.5 (+2.7, -2.5) x 10^21 cm^-2, at a\nredshift of 2.366, in addition to the Galactic value of 1.4 x 10^21\ncm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index\nof 2.07 (+0.13, -0.12) and a best-fitting absorption column consistent\nwith the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV\nflux conversion factor deduced from this spectrum  is 3.4 x 10^-11 (4.7\nx 10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nGalactic foreground: 1.4 x 10^21 cm^-2\nIntrinsic column:    1.4 (+2.2, -0.0) x 10^21 cm^-2 at z=2.366\nPhoton index:\t     2.07 (+0.13, -0.12)\n\nIf the light curve continues to decay with a power-law decay index of\n1.18, the count rate at T+24 hours will be 3.3 x 10^-3 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x\n10^-13 (1.6 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/00755354.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 21182,
  "createdOn": 1496325338000,
  "email": "pae9@leicester.ac.uk",
  "subject": "GRB 170531B: Swift-XRT refined Analysis",
  "submitter": "Phil Evans at U of Leicester  <pae9@leicester.ac.uk>",
  "eventId": "GRB 170531B"
}