{
  "bibcode": "2025GCN.40416....1P",
  "eventId": "GRB 250509A",
  "createdOn": 1746867776078,
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "subject": "GRB 250509A: Swift-XRT refined Analysis",
  "body": "K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M.G.\nBernardini (INAF-OAB), E. Ambrosi  (INAF-IASFPA) , M. Capalbi\n(INAF-IASFPA), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU)\nand P.A. Evans report on behalf of the Swift-XRT team:\n\nWe have analysed 6.8 ks of XRT data for GRB 250509A, from 63 s to 28.3\nks after the   trigger. The data comprise 156 s in Windowed Timing (WT)\nmode (the first 6 s were taken while Swift was slewing) with the\nremainder in Photon Counting (PC) mode. \n\nThe light curve can be modelled with a series of power-law decays. The\ninitial decay index is alpha=3.20 (+/-0.09). At T+349 s  the decay\nflattens to an alpha of -1.1 (+0.7, -0.4) before breaking again at\nT+1377 s to a final decay with index alpha=1.44 (+0.10, -0.09).\n\nA spectrum formed from the WT mode data can be fitted with an absorbed\npower-law with a photon spectral index\tof 2.02 (+0.11, -0.10). The\nbest-fitting absorption column is  6.6 (+/-0.7) 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 2.13 (+/-0.15) and a\nbest-fitting absorption column of 6.0 (+1.0, -0.9) 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 (7.5 x 10^-11) erg cm^-2\ncount^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     6.0 (+1.0, -0.9) x 10^21 cm^-2\nGalactic foreground: 2.0 x 10^20 cm^-2\nExcess significance: 10.7 sigma\nPhoton index:\t     2.13 (+/-0.15)\n\nIf the light curve continues to decay with a power-law decay index of\n1.44, the count rate at T+24 hours will be 9.5 x 10^-3 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.9 x\n10^-13 (7.1 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/01311764.\n\nThis circular is an official product of the Swift-XRT team.\n",
  "submittedHow": "email",
  "circularId": 40416
}