{
  "bibcode": "2018GCN.23156....1D",
  "body": "A. D'Ai (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows\n(PSU), A. Tohuvavohu (PSU), A.P. Beardmore (U. Leicester), P.A. Evans\n(U. Leicester), S.L. Gibson (U. Leicester), A. Melandri (INAF-OAB), P.\nD'Avanzo (INAF-OAB) and F.E. Marshall report on behalf of the Swift-XRT\nteam:\n\nWe have analysed 8.1 ks of XRT data for GRB 180818B (Marshall et al.\nGCN Circ. 23149), from 210 s to 52.8 ks after the  BAT trigger. The\ndata comprise 205 s in Windowed Timing (WT) mode with the remainder in\nPhoton Counting (PC) mode. The enhanced XRT position for this burst was\ngiven by Beardmore et al. (GCN Circ. 23151).\n\nThe late-time light curve (from T0+4.5 ks) can be modelled with a\npower-law decay with a decay index of alpha=0.86 (+/-0.07).\n\nA spectrum formed from the WT mode data can be fitted with an absorbed\npower-law with a photon spectral index\tof 3.47 (+0.12, -0.11). The\nbest-fitting absorption column is  2.89 (+0.25, -0.23) x 10^21 cm^-2,\nin excess of the Galactic value of 1.3 x 10^21 cm^-2 (Willingale et al.\n2013). The PC mode spectrum has a photon index of 1.77 (+0.07, -0.06)\nand a best-fitting absorption column consistent with the Galactic\nvalue. The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 3.8 x 10^-11 (4.5 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     1.27 (+0.11, -0.00) x 10^21 cm^-2\nGalactic foreground: 1.3 x 10^21 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.77 (+0.07, -0.06)\n\nIf the light curve continues to decay with a power-law decay index of\n0.86, the count rate at T+24 hours will be 0.030 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x\n10^-12 (1.3 x 10^-12) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00853882.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 23156,
  "createdOn": 1534655480000,
  "email": "pae9@leicester.ac.uk",
  "subject": "GRB 180818B: Swift-XRT refined Analysis",
  "submitter": "Phil Evans at U of Leicester  <pae9@leicester.ac.uk>",
  "eventId": "GRB 180818B"
}