{
  "circularId": 40996,
  "submittedHow": "email",
  "body": "M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), J.A. Kennea (PSU), D.N.\nBurrows (PSU), M.A. Williams (PSU), A.P. Beardmore (U. Leicester), J.P.\nOsborne (U. Leicester), A. D'Ai (INAF-IASFPA) and P.A. Evans (U.\nLeicester) reports on behalf of the Swift-XRT team:\n\nSwift has performed follow-up observations of the SVOM detected burst\nGRB250706B (GCN 40989), collecting 1.6 ks of XRT data from 1.4 ks to\n3.0 ks after the  SVOM/ECLAIRs trigger. The data are entirely in Photon\nCounting (PC) mode. A candidate X-ray counterpart has been found with a\nposition RA, Dec = 41.2296, -50.0604 which is equivalent to:\n\nRA (J2000): 02 44 55.11\nDec(J2000): -50 03 37.5\n\nwith an uncertainty of 3.5 arcsec (radius, 90% confidence).\n\nWe consider the X-ray source the afterglow of GRB 250706B, even if this\nposition appears only marginally consistent with the optical\ncounterpart detected by TRT (Zhu et al., GCN 40991) and from SVOM/VT\n(GCN 40992).\n\nThe light curve can be modelled with an initial power-law decay with an\nindex of alpha=0.93 (+0.17, -0.21), followed by a break at T+2679 s to\nan alpha of 8.0 (+0.0, -1.4).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.50 (+/-0.07). The\nbest-fitting absorption column is  1.11 (+0.24, -0.23) x 10^21 cm^-2,\nin excess of the Galactic value of 2.8 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 4.3 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     1.11 (+0.24, -0.23) x 10^21 cm^-2\nGalactic foreground: 2.8 x 10^20 cm^-2\nExcess significance: 5.9 sigma\nPhoton index:\t     1.50 (+/-0.07)\n\nIf the light curve continues to decay with a power-law decay index of\n8.0, the count rate at T+24 hours will be 1.3 x 10^-11 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.6 x\n10^-22 (6.2 x 10^-22) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00019912.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "createdOn": 1751837639094,
  "subject": "GRB250706B: Swift-XRT counterpart detection",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "bibcode": "2025GCN.40996....1F",
  "eventId": "GRB 250706B"
}