{
  "circularId": 41325,
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "body": "T. Sbarrato (INAF-OAB), D.N. Burrows (PSU), M.A. Williams (PSU), S.\nDichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester),\nP. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB) and P.A. Evans (U.\nLeicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of GRB 250812A. We\nsearched for X-ray sources in  977 s of Photon Counting (PC) mode data.\nThe total exposure at the position of the afterglow (see below) is 977\ns, obtained between T0+6.7 ks and T0+12.4 ks.\n\nAn uncatalogued X-ray source is detected and is above the RASS 3-sigma\nupper limit at this position, and is therefore likely the GRB\nafterglow. Using 1351 s of PC mode data and 2 UVOT images, we find an\nenhanced XRT position (using the XRT-UVOT alignment and matching UVOT\nfield sources to the USNO-B1 catalogue): RA, Dec = 32.80701, -43.16360\nwhich is equivalent to:\n\nRA (J2000): 02h 11m 13.68s\nDec(J2000): -43d 09' 49.0\"\n\nwith an uncertainty of 3.0 arcsec (radius, 90% confidence). The light\ncurve is consistent with a constant source of mean count rate 1.5e-01\nct/sec. A power-law fit gives an index of 0.16 (+0.64, -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 1.97 (+0.40, -0.23). The\nbest-fitting absorption column is  consistent with the Galactic value\nof 1.5 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed\n(unabsorbed) 0.3-10 keV flux conversion factor deduced from this\nspectrum  is 3.1 x 10^-11 (3.2 x 10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     1.5 (+/-6.9) x 10^20 cm^-2\nGalactic foreground: 1.5 x 10^20 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.97 (+0.40, -0.23)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00019994.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/SVOM_FIELD00028.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "eventId": "GRB 250812A",
  "subject": "GRB 250812A: Swift-XRT afterglow detection",
  "bibcode": "2025GCN.41325....1S",
  "createdOn": 1755005685536,
  "submittedHow": "email"
}