{
  "createdOn": 1743353575810,
  "subject": "GRB 250330A: Swift-XRT afterglow detection",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "submittedHow": "email",
  "bibcode": "2025GCN.39947....1O",
  "circularId": 39947,
  "eventId": "GRB 250330A",
  "body": "J.P. Osborne (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai\n(INAF-IASFPA), A. Melandri (INAF-OAR), D.N. Burrows (PSU), M. A.\nWilliams (PSU), S. Dichiara (PSU), K.L. Page (U. Leicester) and P.A.\nEvans (U. Leicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the\nSwift-BAT-detected burst GRB 250330A. We searched for X-ray sources in \n1.4 ks of Photon Counting (PC) mode data. The total exposure at the\nposition of the afterglow (see below) is 5.1 ks, obtained between\nT0+3.9 ks and T0+28.0 ks.\n\nTwo uncatalogued X-ray sources are detected within the estimated\n3-sigma Swift-BAT error region (296 arcsec), of which one (\"Source 1\")\nis above the LSXPS 3-sigma upper limit at this position and fading with\n>3-sigma significance, and is therefore likely the GRB afterglow. Using\n1474 s of PC mode data and 4 UVOT images, we find an enhanced XRT\nposition (using the XRT-UVOT alignment and matching UVOT field sources\nto the USNO-B1 catalogue): RA, Dec = 252.94482, -86.88053 which is\nequivalent to:\n\nRA (J2000): 16h 51m 46.76s\nDec(J2000): -86d 52' 49.9\"\n\nwith an uncertainty of 2.3 arcsec (radius, 90% confidence). This\nposition is 48 arcsec from the Swift-BAT position. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=1.01 (+0.15, -0.14).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.3 (+0.4, -0.3). The\nbest-fitting absorption column is  4.9 (+1.9, -1.5) x 10^21 cm^-2, in\nexcess of the Galactic value of 1.2 x 10^21 cm^-2 (Willingale et al.\n2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 3.6 x 10^-11 (7.0 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     4.9 (+1.9, -1.5) x 10^21 cm^-2\nGalactic foreground: 1.2 x 10^21 cm^-2\nExcess significance: 3.9 sigma\nPhoton index:\t     2.3 (+0.4, -0.3)\n\nIf the light curve continues to decay with a power-law decay index of\n1.01, the count rate at T+24 hours will be 7.8 x 10^-3 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x\n10^-13 (5.4 x 10^-13) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/01299776.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/01299776.\n\nThis circular is an official product of the Swift-XRT team.\n\n"
}