{
  "createdOn": 1762510332040,
  "submittedHow": "email",
  "circularId": 42611,
  "subject": "GRB 251106A: Swift-XRT afterglow detection",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "bibcode": "2025GCN.42611....1D",
  "body": "A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), J.A. Kennea (PSU), D.N.\nBurrows (PSU), S. Lanava (PSU), A.P. Beardmore (U. Leicester), J.P.\nOsborne (U. Leicester), A. Melandri (INAF-OAR) and P.A. Evans (U.\nLeicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of GRB 251106A. We\nsearched for X-ray sources in  2.0 ks of Photon Counting (PC) mode\ndata. The total exposure at the position of the afterglow (see below)\nis 2.0 ks, obtained between T0+4.0 ks and T0+9.9 ks.\n\nThree uncatalogued X-ray sources are detected, of which one (\"Source\n1\") is above the RASS 3-sigma upper limit at this position, and is\ntherefore likely the GRB afterglow. Using 2020 s of PC mode data and 2\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 = 234.46260, +63.36779 which is equivalent to:\n\nRA (J2000): 15h 37m 51.02s\nDec(J2000): +63d 22' 04.0\"\n\nwith an uncertainty of 2.6 arcsec (radius, 90% confidence). This\nposition is consistent with the optical afterglow candidate identified\nby Angulo et al. (GCN 42607).\n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=0.87 (+0.33, -0.29).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.96 (+0.23, -0.22). The\nbest-fitting absorption column is  1.1 (+0.6, -0.5) x 10^21 cm^-2, in\nexcess of the Galactic value of 1.7 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 3.2 x 10^-11 (3.9 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     1.1 (+0.6, -0.5) x 10^21 cm^-2\nGalactic foreground: 1.7 x 10^20 cm^-2\nExcess significance: 2.8 sigma\nPhoton index:\t     1.96 (+0.23, -0.22)\n\nIf the light curve continues to decay with a power-law decay index of\n0.87, the count rate at T+24 hours will be 0.038 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x\n10^-12 (1.5 x 10^-12) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis are available at\nhttps://www.swift.ac.uk/xrt_products/03000182.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/SVOM/SVOM_FIELD00048.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "eventId": "GRB 251106A"
}