{
  "body": "M.G. Bernardini (INAF-OAB), J. D. Gropp (PSU), S. Dichiara (PSU), J.A.\nKennea (PSU), P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester),\nK.L. Page (U. Leicester), E. Ambrosi  (INAF-IASFPA) , M. Capalbi\n(INAF-IASFPA) report on behalf of the Swift-XRT team:\n\nWe have analysed 6.3 ks of XRT data for GRB 230618A, from 118 s to 30.0\nks after the  BAT trigger. The data comprise 157 s in Windowed Timing\n(WT) mode (the first 9 s were taken while Swift was slewing) with the\nremainder in Photon Counting (PC) mode. The best available XRT position\n (using the promptly downlinked event data, the XRT-UVOT alignment and\nmatching UVOT field sources to the USNO-B1 catalogue) is RA, Dec =\n196.60531, 3.30238 which is equivalent to:\n\nRA (J2000): 13 06 25.27\nDec(J2000): +03 18 08.6\n\nwith an uncertainty of 1.9 arcsec (radius, 90% confidence).\n\nThe light curve can be modelled with a series of power-law decays. The\ninitial decay index is alpha=1.10 (+0.13, -0.10). At T+594 s  the decay\nsteepens to an alpha of 2.90 (+0.40, -0.23) before breaking again at\nT+3047 s to a final decay with index alpha=0.29 (+0.68, -0.29).\n\nA spectrum formed from the WT mode data can be fitted with an absorbed\npower-law with a photon spectral index\tof 1.55 (+0.13, -0.12). The\nbest-fitting absorption column is  1.4 (+0.5, -0.4) x 10^21 cm^-2, in\nexcess of the Galactic value of 2.1 x 10^20 cm^-2 (Willingale et al.\n2013). The PC mode spectrum has a photon index of 1.73 (+/-0.17) and a\nbest-fitting absorption column of 2.1 (+0.7, -0.6) x 10^21 cm^-2. The\ncounts to observed (unabsorbed) 0.3-10 keV flux conversion factor\ndeduced from this spectrum  is 4.1 x 10^-11 (5.2 x 10^-11) erg cm^-2\ncount^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     2.1 (+0.7, -0.6) x 10^21 cm^-2\nGalactic foreground: 2.1 x 10^20 cm^-2\nExcess significance: 5.0 sigma\nPhoton index:\t     1.73 (+/-0.17)\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 8.8 x 10^-3 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x\n10^-13 (4.5 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/01175438.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "subject": "GRB 230618A: Swift-XRT refined Analysis",
  "circularId": 34007,
  "bibcode": "2023GCN.34007....1B",
  "createdOn": 1687103805151,
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "eventId": "GRB 230618A"
}