{
  "subject": "GRB 250901A: Swift-XRT afterglow detection",
  "submittedHow": "email",
  "eventId": "GRB 250901A",
  "bibcode": "2025GCN.41645....1O",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "body": "J.P. Osborne (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo\n(INAF-OAB), M.G. Bernardini (INAF-OAB), D.N. Burrows (PSU), S. Lanava\n(PSU), S. Dichiara (PSU), K.L. Page (U. Leicester) and P.A. Evans (U.\nLeicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of GRB 250901A. We\nsearched for X-ray sources in  754 s of Photon Counting (PC) mode data.\nThe total exposure at the position of the afterglow (see below) is 754\ns, obtained between T0+2.6 ks and T0+3.3 ks.\n\nAn uncatalogued X-ray source is detected and is above the LSXPS 3-sigma\nupper limit at this position, and is therefore likely the GRB\nafterglow.  The position of this source is RA, Dec=356.1837, -15.7410\nwhich is equivalent to:\n\nRA (J2000): 23h 44m 44.08s\nDec(J2000): -15d 44' 27.7\"\n\nwith an uncertainty of 3.9 arcsec (radius, 90% confidence). \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=3.2 (+1.8, -3.0).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.9 (+0.5, -0.4). The\nbest-fitting absorption column is  6 (+12, -4) x 10^20 cm^-2,\nconsistent with the Galactic value of 1.9 x 10^20 cm^-2 (Willingale et\nal. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux\nconversion factor deduced from this spectrum  is 3.5 x 10^-11 (4.0 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     6 (+12, -4) x 10^20 cm^-2\nGalactic foreground: 1.9 x 10^20 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.9 (+0.5, -0.4)\n\nIf the light curve continues to decay with a power-law decay index of\n3.2, the count rate at T+24 hours will be 2.9 x 10^-6 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x\n10^-16 (1.1 x 10^-16) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/03000066.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/SVOM_FIELD00033.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "createdOn": 1756766016843,
  "circularId": 41645
}