{
  "bibcode": "2025GCN.40361....1B",
  "submittedHow": "email",
  "submitter": "P.A. Evans at U. Leicester <pae9@leicester.ac.uk>",
  "body": "A.P. Beadmore, K.L. Page and P.A. Evans (U. Leicester) report on behalf\nof the Swift-XRT team,\n\nSwift-XRT has observed the location of the SVOM and Fermi-detected GRB 250506A\n(GCN Circs. 40355, 40358), gathering 1.7 ks of data between 1.5 ks and 13.7ks\nafter the Fermi trigger. The observations are centred on the SVOM/ECLAIRs\nposition.\n\nWe find a single, uncatalogued X-ray source with an enhanced XRT position of RA,\nDec = 219.64184, 29.21742 which is equivalent to:\n\nRA  (J2000) = 14h 38m 34.04s\nDec (J2000) = +29° 13′ 02.7\n\nwith an uncertainty of 4.6 arcsec (radius, 90% confidence). This source shows\nevidence for fading at the 2.2-sigma level.\n\nAs noted by Liu et al. (GCN Circ. 50360), this is spatially consistent with an\noptical source which Gaia classifies as a \"quasar with 100.00% probability”\n(Gaia DR3 1281175103580872064). The source also appears in the ALLWISEAGN\ncatalogue (Secrest et al., 2015, ApJS, 221, 12).\n\nThe XRT observations show a mean count-rate of 2.9 (+/- 0.007) ct sec and the\nspectrum can be modelled with an absorbed power-law with photon index 1.8 (+0.8,\n-0.5). The intrinsic absorption is <1.8e21 cm^-2, in addition to the Galactic\ncolumn of 1.44e20 cm^-2. Using these paramters the mean 0.3-10 keV flux is 8.2\n(+1.8, -1.7)e-13 erg cm^-2 s^-1.\n\nNo other X-ray sources are detected, to an upper limit of ~6e-3 ct/sec which\ncorresponds to 2.6e-13 erg cm^-2 s^-1 (0.3-10 keV) assuming a standard power-law\nspectrum (photon index 1.7, absorption column 3e20 cm^-2).\n\nGiven the lack of other sources and how early the XRT observations started,\ncombined with the apparent fading of the XRT afterglow, we consider it likely\nthat this source is the counterpart to the Fermi and SVOM triggers; however,\nfollow up observations are encouraged to determine the nature of this event.\n\nAutomated analysis of the observations is available at https://www.swift.ac.uk/SVOM/SVOM_FIELD00014/.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 40361,
  "eventId": "GRB 250506A",
  "subject": "GRB 250506A: Swift XRT observations and possible counterpart detection",
  "createdOn": 1746525788806
}