{
  "eventId": "GRB 250903A",
  "body": "E. Ambrosi  (INAF-IASFPA) , M. Capalbi (INAF-OAR), J.A. Kennea (PSU),\nD.N. Burrows (PSU), S. Lanava (PSU), A.P. Beardmore (U. Leicester),\nJ.P. Osborne (U. Leicester), M. Perri (SSDC & INAF-OAR) and P.A. Evans\n(U. Leicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of GRB 250903A. We\nsearched for X-ray sources in  2.4 ks of Photon Counting (PC) mode\ndata. The total exposure at the position of the afterglow (see below)\nis 6.3 ks, obtained between T0+12.7 ks and T0+344.1 ks.\n\nFour uncatalogued X-ray sources are detected within the estimated\n3-sigma SVOM/ECLAIRs error region (509 arcsec), of which one (\"Source\n1\") is fading with >3-sigma significance and its position is consistent\nwith the optical afterglow (An et al., GCN 41679). Therefore, this is\nthe GRB afterglow. Using 2370 s of PC mode data and 3 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 =\n356.79798, -75.96644 which is equivalent to:\n\nRA (J2000): 23h 47m 11.52s\nDec(J2000): -75d 57' 59.2\"\n\nwith an uncertainty of 3.0 arcsec (radius, 90% confidence).\n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=0.9 (+/-0.9).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.95 (+0.73, -0.21). The\nbest-fitting absorption column is  consistent with the Galactic value\nof 4.7 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed\n(unabsorbed) 0.3-10 keV flux conversion factor deduced from this\nspectrum  is 3.4 x 10^-11 (3.6 x 10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     4 (+/-10) x 10^20 cm^-2\nGalactic foreground: 4.7 x 10^20 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.95 (+0.73, -0.21)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/03000070.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/SVOM/SVOM_FIELD00034.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "bibcode": "2025GCN.41746....1A",
  "createdOn": 1757324831434,
  "subject": "GRB 250903A: Swift-XRT afterglow detection",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "circularId": 41746,
  "submittedHow": "email"
}