{
  "createdOn": 1763646804677,
  "circularId": 42782,
  "eventId": "GRB 251118A",
  "bibcode": "2025GCN.42782....1O",
  "subject": "GRB 251118A: Swift-XRT afterglow detection",
  "submittedHow": "email",
  "body": "J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M.G. Bernardini\n(INAF-OAB), E. Ambrosi\t(INAF-IASFPA) , M. Capalbi (INAF-OAR), D.N.\nBurrows (PSU), S. Lanava (PSU), S. Dichiara (PSU) and P.A. Evans (U.\nLeicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the\nFermi/LAT-detected burst GRB 251118A. We searched for X-ray sources in \n3.0 ks of Photon Counting (PC) mode data. The total exposure at the\nposition of the afterglow (see below) is 5.4 ks, obtained between\nT0+31.3 ks and T0+108.8 ks.\n\nTwo uncatalogued X-ray sources are detected, of which one (\"Source 1\")\nis above the RASS 3-sigma upper limit at this position, and is\ntherefore likely the GRB afterglow. Using 3939 s of PC mode data and 7\nUVOT images, we find an enhanced XRT position (using the XRT-UVOT\nalignment and matching UVOT field sources to the USNO-B1 catalogue):\nRA, Dec = 160.00418, -34.52259 which is equivalent to:\n\nRA (J2000): 10h 40m 01.00s\nDec(J2000): -34d 31' 21.3\"\n\nwith an uncertainty of 2.0 arcsec (radius, 90% confidence). This\nposition is 5.0 arcmin from the Fermi/LAT position. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=1.08 (+0.18, -0.16).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.73 (+0.16, -0.15). The\nbest-fitting absorption column is  2.2 (+0.7, -0.6) x 10^21 cm^-2, in\nexcess of the Galactic value of 8.3 x 10^20 cm^-2 (Willingale et al.\n2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 4.1 x 10^-11 (5.2 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     2.2 (+0.7, -0.6) x 10^21 cm^-2\nGalactic foreground: 8.3 x 10^20 cm^-2\nExcess significance: 3.9 sigma\nPhoton index:\t     1.73 (+0.16, -0.15)\n\nThe results of the XRT-team automatic analysis are available at\nhttps://www.swift.ac.uk/xrt_products/00021890.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/00021890.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>"
}