{
  "body": "A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), A. Melandri\n(INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea\n(PSU), D.N. Burrows (PSU), S. Lanava (PSU) and P.A. Evans (U.\nLeicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has conducted further observations of the field of the\nMAXI-detected burst GRB 250830B. The observations now extend from\nT0+11.1 ks to T0+223.0 ks and have a total exposure time of 6.0 ks. .\nThe source previously reported, \"Source 1\", is fading with 2.5 sigma\nsignificance and thus is believed to be the GRB afterglow. Using 2349 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 = 105.74037, -64.85289 which is equivalent\nto:\n\nRA (J2000): 07h 02m 57.69s\nDec(J2000): -64d 51' 10.4\"\n\nwith an uncertainty of 3.0 arcsec (radius, 90% confidence). This\nposition is 2.3 arcmin from the EP position. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=0.43 (+0.15, -0.16).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.4 (+0.8, -0.5). The\nbest-fitting absorption column is  1.5 (+6.0, -0.4) x 10^21 cm^-2,\nconsistent with the Galactic value of 1.1 x 10^21 cm^-2 (Willingale et\nal. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux\nconversion factor deduced from this spectrum  is 5.4 x 10^-11 (6.1 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     1.5 (+6.0, -0.4) x 10^21 cm^-2\nGalactic foreground: 1.1 x 10^21 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.4 (+0.8, -0.5)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00021858.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/00021858.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "submittedHow": "email",
  "eventId": "GRB 250830B",
  "bibcode": "2025GCN.41658....1B",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "subject": "GRB 250830B: Swift-XRT afterglow detection",
  "circularId": 41658,
  "createdOn": 1756823513592
}