{
  "circularId": 42891,
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "createdOn": 1764419358858,
  "submittedHow": "email",
  "bibcode": "2025GCN.42891....1P",
  "body": "K.L. Page (U. Leicester), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti\n(INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU),\nA.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester) and P.A.\nEvans (U. Leicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the\nSVOM/ECLAIRs-detected burst GRB 251129A. We searched for X-ray sources\nin  732 s of Photon Counting (PC) mode data. The total exposure at the\nposition of the afterglow (see below) is 2.3 ks, obtained between\nT0+16.5 ks and T0+28.5 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 2336 s of PC mode data and 4 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 = 224.33074, +79.29312\nwhich is equivalent to:\n\nRA (J2000): 14h 57m 19.38s\nDec(J2000): +79d 17' 35.2\"\n\nwith an uncertainty of 2.1 arcsec (radius, 90% confidence). This\nposition is 26 arcsec from the SVOM/ECLAIRs position. The position is\nconsistent with the optical afterglow (GCNs 42880, 42882, 42883, 42885,\n42887) and the EP X-ray transient (GCN 42884).\n\nThe light curve can be modelled can be modelled with a power-law decay\nwith a decay index of alpha=6.23 (+0.32, -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 2.71 (+0.22, -0.20). The\nbest-fitting absorption column is  7.3 (+3.7, -3.3) x 10^20 cm^-2, in\nexcess of the Galactic value of 2.9 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 2.6 x 10^-11 (3.7 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     7.3 (+3.7, -3.3) x 10^20 cm^-2\nGalactic foreground: 2.9 x 10^20 cm^-2\nExcess significance: 2.2 sigma\nPhoton index:\t     2.71 (+0.22, -0.20)\n\nIf the light curve continues to decay with a power-law decay index of\n6.2, the count rate at T+24 hours will be 1.1 x 10^-4 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x\n10^-15 (4.2 x 10^-15) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis are available at\nhttps://www.swift.ac.uk/xrt_products/00021892.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/00021892.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "subject": "GRB 251129A: Swift-XRT afterglow detection",
  "eventId": "GRB 251129A"
}