{
  "createdOn": 1746693758491,
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "circularId": 40394,
  "submittedHow": "email",
  "eventId": "GRB 250506A",
  "subject": "GRB 250506A: Swift-XRT afterglow detection",
  "bibcode": "2025GCN.40394....1B",
  "body": "D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.\nLeicester), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G.\nBernardini (INAF-OAB), M. A. Williams (PSU), S. Dichiara (PSU) and P.A.\nEvans (U. Leicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has conducted further observations of the field of the\nSVOM/MXT-detected burst GRB 250506A. The observations now extend from\nT0+63.4 ks to T0+138.3 ks and have a total exposure time of 6.3 ks. The\nsource previously reported, \"Source 1\", is fading with 2.8 sigma\nsignificance and thus is believed to be the GRB afterglow. Using 2772 s\nof PC mode data and 2 UVOT images, we find an enhanced XRT position\n(using the XRT-UVOT alignment and matching UVOT field sources to the\nUSNO-B1 catalogue): RA, Dec = 219.28269, +28.87835 which is equivalent\nto:\n\nRA (J2000): 14h 37m 07.85s\nDec(J2000): +28d 52' 42.0\"\n\nwith an uncertainty of 2.6 arcsec (radius, 90% confidence). This\nposition is 84 arcsec from the SVOM/MXT position. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=1.7 (+0.5, -0.4).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.74 (+0.34, -0.21). The\nbest-fitting absorption column is  2.1 (+7.8, -0.4) x 10^20 cm^-2,\nconsistent with the Galactic value of 1.6 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 3.8 x 10^-11 (3.9 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     2.1 (+7.8, -0.4) x 10^20 cm^-2\nGalactic foreground: 1.6 x 10^20 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.74 (+0.34, -0.21)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00021828.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/00021828.\n\nThis circular is an official product of the Swift-XRT team.\n\n"
}