{
  "circularId": 40336,
  "createdOn": 1746298830061,
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "eventId": "GRB 250502A",
  "submittedHow": "email",
  "body": "S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U.\nLeicester), A.P. Beardmore (U. Leicester), A. D'Ai (INAF-IASFPA), A.\nMelandri (INAF-OAR), T. Sbarrato (INAF-OAB) and P.A. Evans (U.\nLeicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the\nSVOM/ECLAIRs-detected burst GRB 250502A (GCN 40313). We searched for\nX-ray sources in  4.6 ks of Photon Counting (PC) mode data. The total\nexposure at the position of the afterglow (see below) is 4.6 ks,\nobtained between T0+72.1 ks and T0+105.5 ks.\n\nAn uncatalogued X-ray source is detected consistent with the\nSVOM/COLIBRI error region (GCN 40315) and is believed to be the\nafterglow. Using 2113 s of PC mode data and 2 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 = 206.52396, -10.75879\nwhich is equivalent to:\n\nRA (J2000): 13h 46m 05.75s\nDec(J2000): -10d 45' 31.6\"\n\nwith an uncertainty of 4.5 arcsec (radius, 90% confidence). This\nposition is 2.3 arcsec from the SVOM/COLIBRI position.\tThe source has\na mean count rate of 1.3e-02 ct/sec; we cannot determine at the present\ntime whether it is fading. However, given the coincidence in position\nwith the optical counterpart (GCNs 40315, 40319, 40320, 40322, 40330,\n40331, 40333, 40334) at a redshift of 2.163 (GCN 40328), we confirm\nthis as the X-ray afterglow.\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.4 (+1.6, -0.7). The\nbest-fitting absorption column is  consistent with the Galactic value\nof 5.0 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 3.1 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     5 (+/-31) x 10^20 cm^-2\nGalactic foreground: 5.0 x 10^20 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     2.4 (+1.6, -0.7)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00021827.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/00021827.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "bibcode": "2025GCN.40336....1D",
  "subject": "GRB 250502A: Swift-XRT afterglow detection"
}