{
  "subject": "GRB 150428B: Swift-XRT refined Analysis",
  "eventId": "GRB 150428B",
  "bibcode": "2015GCN.17781....1D",
  "createdOn": 1430235423000,
  "circularId": 17781,
  "submitter": "Phil Evans at U of Leicester  <pae9@leicester.ac.uk>",
  "email": "pae9@leicester.ac.uk",
  "body": "P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), J.A. Kennea (PSU), B.\nSbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), P.A. Evans (U.\nLeicester), A. Amaral-Rogers (U. Leicester), J.P. Osborne (U.\nLeicester), M. de Pasquale (INAF-IASFPA) and K.L. Page report on behalf\nof the Swift-XRT team:\n\nWe have analysed 8.3 ks of XRT data for GRB 150428B (Page et al. GCN\nCirc. 17766), from 100 s to 34.4 ks after the  BAT trigger. The data\ncomprise 187 s in Windowed Timing (WT) mode (the first 6 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 Beardmore\net al. (GCN Circ. 17773).\n\nThe light curve can be modelled with a series of power-law decays. The\ninitial decay index is alpha=2.31 (+0.16, -0.20). At T+170 s  the decay\nsteepens to an alpha of 4.92 (+0.15, -0.16) before breaking again at\nT+618 s to a final decay with index alpha=0.29 (+/-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 1.93 (+0.06, -0.05). The\nbest-fitting absorption column is  3.51 (+0.28, -0.25) x 10^21 cm^-2,\nconsistent with the Galactic value of 3.3 x 10^21 cm^-2 (Willingale et\nal. 2013). The PC mode spectrum has a photon index of 1.90 (+0.16,\n-0.08) and a best-fitting absorption column consistent with the\nGalactic value. The counts to observed (unabsorbed) 0.3-10 keV flux\nconversion factor deduced from this spectrum  is 3.9 x 10^-11 (5.6 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     3.3 (+0.3, -0.0) x 10^21 cm^-2\nGalactic foreground: 3.3 x 10^21 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.90 (+0.16, -0.08)\n\nIf the light curve continues to decay with a power-law decay index of\n0.29, the count rate at T+24 hours will be 0.027 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x\n10^-12 (1.5 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/00639278.\n\nThis circular is an official product of the Swift-XRT team."
}