{
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "circularId": 41885,
  "subject": "GRB 250919A: Swift-XRT afterglow detection",
  "submittedHow": "email",
  "body": "J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Ferro\n(INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Lanava\n(PSU), S. Dichiara (PSU), J.A. Kennea (PSU), A.P. Beardmore (U.\nLeicester) and P.A. Evans (U. Leicester) reports on behalf of the\nSwift-XRT team:\n\nSwift-XRT has performed follow-up observations of GRB\n250919A/EP250919a. We searched for X-ray sources in  1.7 ks of Photon\nCounting (PC) mode data. The total exposure at the position of the\nafterglow (see below) is 1.7 ks, obtained between T0+21.4 ks and\nT0+23.1 ks, taking T0 as the Fermi trigger time.\n\nAn uncatalogued X-ray source is detected within the estimated 3-sigma\nEinstein Probe/WXT error region (35 arcsec), also consistent with the\nFermi-LAT error region, and is above the RASS 3-sigma upper limit at\nthis position, and is therefore likely the GRB afterglow. Using 1732 s\nof PC mode data and 1 UVOT image, we find an enhanced XRT position\n(using the XRT-UVOT alignment and matching UVOT field sources to the\nUSNO-B1 catalogue): RA, Dec = 298.55415, -48.83775 which is equivalent\nto:\n\nRA (J2000): 19h 54m 13.00s\nDec(J2000): -48d 50' 15.9\"\n\nwith an uncertainty of 2.5 arcsec (radius, 90% confidence). The light\ncurve is consistent with a constant source of mean count rate 2.3e+00\nct/sec. A power-law fit gives an index of 1.0 (+1.9, -1.0). However,\ngiven the position of this X-ray saource is consistent with the optical\ncounterpart with a measured redshift of 1.145 (GCN 41883), as well as\nbeing above the RASS 3-sigma upper limit, we believe this to be the\nX-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 1.52 (+0.13, -0.11). The\nbest-fitting absorption column is  6.7 (+4.1, -1.6) x 10^20 cm^-2,\nconsistent with the Galactic value of 5.1 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 4.5 x 10^-11 (4.8 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     6.7 (+4.1, -1.6) x 10^20 cm^-2\nGalactic foreground: 5.1 x 10^20 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.52 (+0.13, -0.11)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/03000101.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/EP/EP_FIELD00075.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "bibcode": "2025GCN.41885....1O",
  "eventId": "GRB 250919A",
  "createdOn": 1758290609844
}