{
  "circularId": 40950,
  "createdOn": 1751637605472,
  "subject": "GRB 250704A: Swift-XRT afterglow detection",
  "bibcode": "2025GCN.40950....1O",
  "eventId": "GRB 250704A",
  "submittedHow": "email",
  "body": "J.P. Osborne (U. Leicester), B. Sbarufatti (INAF-OAB), C. Salvaggio\n(INAF-OAB), M. Ferro (INAF-OAB), M.A. Williams (PSU), S. Dichiara\n(PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.\nLeicester) and P.A. Evans (U. Leicester) reports on behalf of the\nSwift-XRT team:\n\nWe have analysed 2.0 ks of XRT data for GRB 250704A, from 2.9 ks to 8.7\nks after the  SVOM/ECLAIRs trigger. The data are entirely in Photon\nCounting (PC) mode. A bright, fading source is detected inside the\nSVOM/ECLAIRs error region (Cao et al., GCN 40934), which we propose as\nthe GRB afterglow. Using 2029 s of PC mode data and 2 UVOT images, we\nfind an enhanced XRT position (using the XRT-UVOT alignment and\nmatching UVOT field sources to the USNO-B1 catalogue): RA, Dec =\n17.19414, -17.32397 which is equivalent to:\n\nRA (J2000): 01h 08m 46.59s\nDec(J2000): -17d 19' 26.3\"\n\nwith an uncertainty of 2.8 arcsec (radius, 90% confidence).\n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=1.3 (+0.6, -0.5).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.03 (+0.31, -0.28). The\nbest-fitting absorption column is  6.0 (+6.7, -4.5) x 10^20 cm^-2,\nconsistent with the Galactic value of 1.6 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 3.2 x 10^-11 (3.8 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     6.0 (+6.7, -4.5) x 10^20 cm^-2\nGalactic foreground: 1.6 x 10^20 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     2.03 (+0.31, -0.28)\n\nIf the light curve continues to decay with a power-law decay index of\n1.3, the count rate at T+24 hours will be 2.4 x 10^-3 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.9 x\n10^-14 (9.1 x 10^-14) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00019907.\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>"
}