{
  "body": "V. D'Elia (ASI-SSDC), E. Ambrosi (INAF/IASFPA), K.L. Page (U\nLeicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows\n(PSU), M. A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U.\nLeicester), A. D'Ai (INAF-IASFPA) and P.A. Evans (U. Leicester) reports\non behalf of the Swift-XRT team:\n\nWe have analysed 1.7 ks of XRT data for the Einstein Probe/WXT-detected\nburst GRB 250427A, from 22.9 ks to 28.9 ks after the  Einstein\nProbe/WXT trigger. The data are entirely in Photon Counting (PC) mode. \n\nWe found an uncatalogued X-ray source within the estimated 3-sigma\nEP-WXT error region. We find an XRT position: RA, Dec = 277.27280,\n+7.5632 which is equivalent\nto:\n\nRA (J2000): 18h 29m 05.47s\nDec (J2000): +07° 33′ 47.6″\n\nwith an uncertainty of 3.9 arcsec (radius, 90% confidence).\nThis position is 6.9 arcsec from the EP-WXT position, and consistent\nwith the X-ray counterpart found in EP/FXT data (Wang et al. GCN 40257)\nand with the optical counterpart reported by Perez-Garcia et al. (GCN\n40259), Becerra et al. (GCN 40261), Ghosh et al. (GCN 40263), Brivio et\nal. (GCN 40264).\n\nThe light curve is consistent with a constant source with a hint of\nfading in the last segment. However, given the corresponding optical\ncounterpart and the measured redshift, we believe this to be the X-ray\nafterglow. Further observations are planned.\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.78 (+0.39, -0.28). The\nbest-fitting absorption column is  consistent with the Galactic value\nof 2.9 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed\n(unabsorbed) 0.3-10 keV flux conversion factor deduced from this\nspectrum  is 4.3 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     2.9 (+/-1.3) x 10^21 cm^-2\nGalactic foreground: 2.9 x 10^21 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.78 (+0.39, -0.28)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00019750.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "circularId": 40268,
  "subject": "GRB 250427A: Swift-XRT refined Analysis",
  "submittedHow": "email",
  "bibcode": "2025GCN.40268....1D",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "createdOn": 1745776183436,
  "eventId": "GRB 250427A"
}