{
  "circularId": 41719,
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "createdOn": 1757064044137,
  "bibcode": "2025GCN.41719....1O",
  "body": "J.P. Osborne (U. Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini\n(INAF-OAB), E. Ambrosi\t(INAF-IASFPA) , S. Lanava (PSU), S. Dichiara\n(PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.\nLeicester) and P.A. Evans report on behalf of the Swift-XRT team:\n\nWe have analysed 7.0 ks of XRT data for GRB 250904A, from 66 s to 45.6\nks after the   trigger. The data comprise 9 s in Windowed Timing (WT)\nmode (taken while Swift was slewing), with the remainder in Photon\nCounting (PC) mode. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=0.85 (+0.05, -0.04).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.05 (+0.27, -0.25). The\nbest-fitting absorption column is  4.4 (+1.4, -1.0) x 10^21 cm^-2,\nconsistent with the Galactic value of 3.3 x 10^21 cm^-2 (Willingale et\nal. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux\nconversion factor deduced from this spectrum  is 4.0 x 10^-11 (6.5 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     4.4 (+1.4, -1.0) x 10^21 cm^-2\nGalactic foreground: 3.3 x 10^21 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     2.05 (+0.27, -0.25)\n\nIf the light curve continues to decay with a power-law decay index of\n0.85, the count rate at T+24 hours will be 7.0 x 10^-3 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 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/01347179.\n\nThis circular is an official product of the Swift-XRT team.\n",
  "submittedHow": "email",
  "subject": "GRB 250904A: Swift-XRT refined Analysis",
  "eventId": "GRB 250904A"
}