{
  "eventId": "GRB 251222A",
  "submittedHow": "email",
  "bibcode": "2025GCN.43213....1B",
  "subject": "GRB 251222A: Swift-XRT afterglow detection",
  "circularId": 43213,
  "body": "A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAR), T. Sbarrato\n(INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows\n(PSU), S. Lanava (PSU), J.P. Osborne (U. Leicester) and P.A. Evans (U.\nLeicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the\nSVOM/ECLAIRs-detected burst GRB 251222A. We searched for X-ray sources\nin  2.0 ks of Photon Counting (PC) mode data. The total exposure at the\nposition of the afterglow (see below) is 2.0 ks, obtained between\nT0+10.7 ks and T0+17.4 ks.\n\nAn uncatalogued X-ray source is detected within the estimated 3-sigma\nSVOM/ECLAIRs error region (49 arcsec) and is above the RASS 3-sigma\nupper limit at this position, and is therefore likely the GRB\nafterglow. Using 1130 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 = 77.19502, -7.21354\nwhich is equivalent to:\n\nRA (J2000): 05h 08m 46.81s\nDec(J2000): -07d 12' 48.7\"\n\nwith an uncertainty of 2.5 arcsec (radius, 90% confidence). This\nposition is 36 arcsec from the SVOM/ECLAIRs position. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=1.9 (+0.8, -0.7).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.92 (+0.31, -0.20). The\nbest-fitting absorption column is  1.05 (+0.91, -0.12) x 10^21 cm^-2,\nconsistent with the Galactic value of 9.3 x 10^20 cm^-2 (Willingale et\nal. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux\nconversion factor deduced from this spectrum  is 3.6 x 10^-11 (4.3 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     1.05 (+0.91, -0.12) x 10^21 cm^-2\nGalactic foreground: 9.3 x 10^20 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.92 (+0.31, -0.20)\n\nIf the light curve continues to decay with a power-law decay index of\n1.9, the count rate at T+24 hours will be 4.7 x 10^-3 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x\n10^-13 (2.0 x 10^-13) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis of the likely afterglow\nare at https://www.swift.ac.uk/ToO_GRBs/00021895/Source1.php.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/00021895.\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>",
  "createdOn": 1766488359105
}