{
  "bibcode": "2018GCN.23266....1M",
  "body": "A. Melandri (INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti\n(INAF-OAB/PSU), D.N. Burrows (PSU), Z. Liu (NAOC / U. Leicester), J.P.\nOsborne (U. Leicester), K.L. Page (U. Leicester), P. D'Avanzo\n(INAF-OAB), V. D'Elia (ASDC) and T.N. Ukwatta report on behalf of the\nSwift-XRT team:\n\nWe have analysed 6.4 ks of XRT data for GRB 180924A (Ukwatta et al. GCN\nCirc. 23261), from 82 s to 52.3 ks after the  BAT trigger. The data\ncomprise 141 s in Windowed Timing (WT) mode with the remainder in\nPhoton Counting (PC) mode. The enhanced XRT position for this burst was\ngiven by Evans et al. (GCN Circ. 23262).\n\nThe light curve can be modelled with an initial power-law decay with an\nindex of alpha=2.23 (+0.11, -0.09), followed by a break at T+485 s to\nan alpha of 0.70 (+0.06, -0.05).\n\nA spectrum formed from the WT mode data can be fitted with an absorbed\npower-law with a photon spectral index\tof 1.96 (+0.14, -0.13). The\nbest-fitting absorption column is  1.0 (+/-0.4) x 10^21 cm^-2, in\nexcess of the Galactic value of 2.0 x 10^20 cm^-2 (Willingale et al.\n2013). The PC mode spectrum has a photon index of 1.88 (+0.20, -0.19)\nand a best-fitting absorption column of 6.9 (+5.2, -4.5) x 10^20 cm^-2.\nThe counts to observed (unabsorbed) 0.3-10 keV flux conversion factor\ndeduced from this spectrum  is 3.5 x 10^-11 (4.0 x 10^-11) erg cm^-2\ncount^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     6.9 (+5.2, -4.5) x 10^20 cm^-2\nGalactic foreground: 2.0 x 10^20 cm^-2\nExcess significance: 1.8 sigma\nPhoton index:\t     1.88 (+0.20, -0.19)\n\nIf the light curve continues to decay with a power-law decay index of\n0.70, the count rate at T+24 hours will be 0.017 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.8 x\n10^-13 (6.6 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/00863186.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 23266,
  "createdOn": 1537863602000,
  "email": "pae9@leicester.ac.uk",
  "subject": "GRB 180924A: Swift-XRT refined Analysis",
  "submitter": "Phil Evans at U of Leicester  <pae9@leicester.ac.uk>",
  "eventId": "GRB 180924A"
}