{
  "eventId": "GRB 250809A",
  "editedOn": 1754919814346,
  "format": "text/plain",
  "subject": "GRB 250809A: Swift-XRT refined Analysis",
  "editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of K.L. Page at U Leicester <klp5@leicester.ac.uk>",
  "bibcode": "2025GCN.41313....1O",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "circularId": 41313,
  "submittedHow": "email",
  "version": 2,
  "createdOn": 1754819056873,
  "body": "J.P. Osborne (U. Leicester), A. D'Ai (INAF-IASFPA), S. Campana\n(INAF-OAB), A. Melandri (INAF-OAR), M.A. Williams (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 6.7 ks of XRT data for GRB 250809A, from 113 s to 51.4\nks after the   trigger. The data comprise 910 s in Windowed Timing (WT)\nmode (the first 8 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 =\n233.00043, -53.29626 which is equivalent to:\n\nRA (J2000): 15h 32m 00.10s\nDec(J2000): -53d 17' 46.5\"\n\nwith an uncertainty of 3.0 arcsec (radius, 90% confidence).\n\nThe light curve can be modelled with an initial rise, with a power-law\nindex of alpha=-1.5 (NaN, NaN), followed by a break at T+281 s\tto an\nalpha of 8.0 (NaN, NaN).\n\nA spectrum formed from the WT mode data can be fitted with an absorbed\npower-law with a photon spectral index\tof 2.41 (+/-0.13). The\nbest-fitting absorption column is  2.05 (+0.18, -0.17) x 10^22 cm^-2,\nin excess of the Galactic value of 8.0 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 4.9 x 10^-11 (1.6 x 10^-10) erg\ncm^-2 count^-1. \n\nA summary of the WT-mode spectrum is thus:\nTotal column:\t     2.05 (+0.18, -0.17) x 10^22 cm^-2\nGalactic foreground: 8.0 x 10^21 cm^-2\nExcess significance: 11.9 sigma\nPhoton index:\t     2.41 (+/-0.13)\n\nIf the light curve continues to decay with a power-law decay index of\n2.96, the count rate at T+24 hours will be 3.7 x 10^-8 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x\n10^-18 (6.1 x 10^-18) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/01340954.\n\nThis circular is an official product of the Swift-XRT team.\n"
}