{
  "circularId": 41108,
  "eventId": "GRB 250716A",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "submittedHow": "email",
  "bibcode": "2025GCN.41108....1W",
  "body": "M.A. Williams (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne\n(U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U.\nLeicester), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), A. Melandri\n(INAF-OAR) and P.A. Evans (U. Leicester) reports on behalf of the\nSwift-XRT team:\n\nSwift-XRT has performed follow-up observations of the\nFermi/GBM-detected burst GRB 250716A. We searched for X-ray sources in \n2.6 ks of Photon Counting (PC) mode data. The total exposure at the\nposition of the afterglow (see below) is 2.6 ks, obtained between\nT0+79.5 ks and T0+91.7 ks.\n\nAn uncatalogued X-ray source is detected within the GOTO error region\nand is above the RASS 3-sigma upper limit at this position, and is\ntherefore likely the GRB afterglow. Using 2790 s of PC mode data and 2\nUVOT images, we find an enhanced XRT position (using the XRT-UVOT\nalignment and matching UVOT field sources to the USNO-B1 catalogue):\nRA, Dec = 66.77171, -46.56649 which is equivalent to:\n\nRA (J2000): 04h 27m 05.21s\nDec(J2000): -46d 33' 59.4\"\n\nwith an uncertainty of 2.7 arcsec (radius, 90% confidence). This\nposition is 1.9 arcsec from the GOTO position. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=2.755 (+0.015, -0.755).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.22 (+0.30, -0.27). The\nbest-fitting absorption column is  6.9 (+6.4, -5.2) x 10^20 cm^-2, in\nexcess of the Galactic value of 1.2 x 10^20 cm^-2 (Willingale et al.\n2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 3.0 x 10^-11 (3.7 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     6.9 (+6.4, -5.2) x 10^20 cm^-2\nGalactic foreground: 1.2 x 10^20 cm^-2\nExcess significance: 1.8 sigma\nPhoton index:\t     2.22 (+0.30, -0.27)\n\nThe results of the XRT-team automatic analysis of the likely afterglow\nare at https://www.swift.ac.uk/ToO_GRBs/00021845/Source1.php.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/00021845.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "createdOn": 1752765163483,
  "subject": "GRB 250716A: Swift-XRT afterglow detection"
}