{
  "bibcode": "2025GCN.39996....1D",
  "body": "P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), M. A. Williams (PSU), S.\nDichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester),\nM.G. Bernardini (INAF-OAB), E. Ambrosi\t(INAF-IASFPA)  and P.A. Evans\nreport on behalf of the Swift-XRT team:\n\nWe have analysed 5.5 ks of XRT data for GRB 250331B, from 321 s to 35.1\nks after the  BAT trigger. The data are entirely in Photon Counting\n(PC) mode. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=1.11 (+0.17, -0.14).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.2 (+0.9, -0.8). The\nbest-fitting absorption column is  2.1 (+1.5, -1.1) x 10^22 cm^-2, in\nexcess of the Galactic value of 6.6 x 10^21 cm^-2 (Willingale et al.\n2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 5.7 x 10^-11 (1.6 x 10^-10) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     2.1 (+1.5, -1.1) x 10^22 cm^-2\nGalactic foreground: 6.6 x 10^21 cm^-2\nExcess significance: 2.2 sigma\nPhoton index:\t     2.2 (+0.9, -0.8)\n\nIf the light curve continues to decay with a power-law decay index of\n1.11, the count rate at T+24 hours will be 7.9 x 10^-4 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.5 x\n10^-14 (1.2 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/01299965.\n\nThis circular is an official product of the Swift-XRT team.\n",
  "subject": "GRB 250331B: Swift-XRT refined Analysis",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "submittedHow": "email",
  "circularId": 39996,
  "eventId": "GRB 250331B",
  "createdOn": 1743493513940
}