{
  "body": "P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), E. Ambrosi \n(INAF-IASFPA) , S. Lanava (PSU), S. Dichiara (PSU), J.A. Kennea (PSU),\nJ.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore\n(U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the\nSwift-XRT team:\n\nSwift-XRT has performed follow-up observations of GRB 251016A. We\nsearched for X-ray sources in  3.5 ks of Photon Counting (PC) mode\ndata. The total exposure at the position of the afterglow (see below)\nis 5.6 ks, obtained between T0+6.6 ks and T0+107.3 ks.\n\nThirty-two uncatalogued X-ray sources are detected, of which one\n(\"Source 1\") is fading with >3-sigma significance, and is therefore\nlikely the GRB afterglow. Using 1190 s of PC mode data and 2 UVOT\nimages, we find an enhanced XRT position (using the XRT-UVOT alignment\nand matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =\n266.30279, -39.56008 which is equivalent to:\n\nRA (J2000): 17h 45m 12.67s\nDec(J2000): -39d 33' 36.3\"\n\nwith an uncertainty of 3.2 arcsec (radius, 90% confidence).\n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=4.53 (+0.08, -3.08).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.4 (+1.0, -0.6). The\nbest-fitting absorption column is  4.8 (+8.8, -0.9) x 10^21 cm^-2,\nconsistent with the Galactic value of 3.9 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 6.4 x 10^-11 (8.0 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     4.8 (+8.8, -0.9) x 10^21 cm^-2\nGalactic foreground: 3.9 x 10^21 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.4 (+1.0, -0.6)\n\nThe results of the XRT-team automatic analysis are available at\nhttps://www.swift.ac.uk/xrt_products/03000141.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/SVOM/SVOM_FIELD00042.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "createdOn": 1760795360818,
  "submittedHow": "email",
  "circularId": 42356,
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "eventId": "GRB 251016A",
  "bibcode": "2025GCN.42356....1D",
  "subject": "GRB 251016A: Swift-XRT afterglow detection"
}