{
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "eventId": "GRB 251025B",
  "circularId": 42445,
  "bibcode": "2025GCN.42445....1B",
  "createdOn": 1761421939320,
  "subject": "GRB 251025B: Swift-XRT afterglow detection",
  "submittedHow": "email",
  "body": "A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), B.\nSbarufatti (INAF-OAB), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB),\nJ.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU) and P.A. Evans\n(U. Leicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of GRB 251025B. We\nsearched for X-ray sources in  1.7 ks of Photon Counting (PC) mode data\nobtained between T0+2.8 ks and T0+4.5 ks.\n\nAn uncatalogued X-ray source is detected within the estimated 3-sigma\nSVOM/ECLAIRs error region (473 arcsec) and is above the RASS 3-sigma\nupper limit at this position, and is therefore likely the GRB\nafterglow. Using 1712 s of PC mode data and 1 UVOT image, we find an\nenhanced XRT position (using the XRT-UVOT alignment and matching UVOT\nfield sources to the USNO-B1 catalogue): RA, Dec = 29.53107, +30.30373\nwhich is equivalent to:\n\nRA (J2000): 01h 58m 07.46s\nDec(J2000): +30d 18' 13.4\"\n\nwith an uncertainty of 2.7 arcsec (radius, 90% confidence). This source\nis 1.25 arcsec from the SVOM/C-GFT position (Wu et al., GCN Circ.\n42438) and 37 arcsec from the SVOM/MXT position (Hussein et al., GCN\nCirc. 42437). At present, the X-ray source shows no evidence of fading.\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.00 (+0.27, -0.25). The\nbest-fitting absorption column is  1.9 (+0.9, -0.8) x 10^21 cm^-2, in\nexcess of the Galactic value of 5.6 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 3.5 x 10^-11 (4.8 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     1.9 (+0.9, -0.8) x 10^21 cm^-2\nGalactic foreground: 5.6 x 10^20 cm^-2\nExcess significance: 2.8 sigma\nPhoton index:\t     2.00 (+0.27, -0.25)\n\nThe results of the XRT-team automatic analysis are available at\nhttps://www.swift.ac.uk/xrt_products/03000153.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/SVOM/SVOM_FIELD00044.\n\nThis circular is an official product of the Swift-XRT team.\n\n"
}