{
  "bibcode": "2021GCN.30576....1T",
  "body": "A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), J.P. Osborne (U.\nLeicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T.\nSbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini\n(INAF-OAB), D.N. Burrows (PSU) and E. Troja report on behalf of the\nSwift-XRT team:\n\nWe have analysed 7.9 ks of XRT data for GRB 210731A (Troja et al. GCN\nCirc. 30568), from 191 s to 60.1 ks after the  BAT trigger. The data\ncomprise 62 s in Windowed Timing (WT) mode (the first 8 s were taken\nwhile Swift was slewing) with the remainder in Photon Counting (PC)\nmode. Using 3002 s of PC mode data and 8 UVOT images, we find an\nenhanced XRT position (using the XRT-UVOT alignment and matching UVOT\nfield sources to the USNO-B1 catalogue): RA, Dec = 300.30466, -28.06096\nwhich is equivalent to:\n\nRA (J2000): 20h 01m 13.12s\nDec(J2000): -28d 03' 39.4\"\n\nwith an uncertainty of 2.2 arcsec (radius, 90% confidence).\n\nThe light curve can be modelled with an initial power-law decay with an\nindex of alpha=3.6 (+/-0.5), followed by a break at T+424 s to an alpha\nof 0.86 (+0.18, -0.19).\n\nA spectrum formed from the WT mode data can be fitted with an absorbed\npower-law with a photon spectral index\tof 3.07 (+0.29, -0.21). The\nbest-fitting absorption column is  1.36 (+0.47, -0.21) x 10^21 cm^-2,\nconsistent with the Galactic value of 1.1 x 10^21 cm^-2 (Willingale et\nal. 2013). The PC mode spectrum has a photon index of 1.97 (+0.26,\n-0.19) and a best-fitting absorption column of 1.35 (+0.77, -0.20) x\n10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux\nconversion factor deduced from this spectrum  is 3.4 x 10^-11 (4.3 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     1.35 (+0.77, -0.20) x 10^21 cm^-2\nGalactic foreground: 1.1 x 10^21 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.97 (+0.26, -0.19)\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 0.022 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.3 x\n10^-13 (9.3 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/01062336.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 30576,
  "createdOn": 1627838667000,
  "email": "pae9@leicester.ac.uk",
  "subject": "GRB 210731A: Swift-XRT refined Analysis",
  "submitter": "Phil Evans at U of Leicester  <pae9@leicester.ac.uk>",
  "eventId": "GRB 210731A"
}