{
  "body": "E. Ambrosi  (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC &\nINAF-OAR), M. A. Williams (PSU), S. Dichiara (PSU), J.A. Kennea (PSU),\nJ.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore\n(U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the\nSwift-XRT team:\n\nWe have analysed 6.8 ks of XRT data for the Swift-BAT-detected burst\nGRB 250321A, from 108 s to 39.4 ks after the  Swift-BAT trigger. The\ndata comprise 9 s in Windowed Timing (WT) mode (taken while Swift was\nslewing), with the remainder in Photon Counting (PC) mode. Using 1662 s\nof PC mode data and 3 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 = 295.09844, +21.04038 which is equivalent\nto:\n\nRA (J2000): 19h 40m 23.62s\nDec(J2000): +21d 02' 25.4\"\n\nwith an uncertainty of 2.6 arcsec (radius, 90% confidence). This\nposition is 72 arcsec from the Swift-BAT position. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=1.13 (+0.11, -0.10).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.2 (+1.3, -1.0). The\nbest-fitting absorption column is  3.7 (+3.4, -2.2) x 10^22 cm^-2, in\nexcess of the Galactic value of 1.1 x 10^22 cm^-2 (Willingale et al.\n2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 7.3 x 10^-11 (2.4 x 10^-10) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     3.7 (+3.4, -2.2) x 10^22 cm^-2\nGalactic foreground: 1.1 x 10^22 cm^-2\nExcess significance: 2.0 sigma\nPhoton index:\t     2.2 (+1.3, -1.0)\n\nIf the light curve continues to decay with a power-law decay index of\n1.13, the count rate at T+24 hours will be 6.9 x 10^-4 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.0 x\n10^-14 (1.6 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/01297508.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "subject": "GRB 250321A: Swift-XRT refined Analysis",
  "createdOn": 1742581410399,
  "eventId": "GRB 250321A",
  "bibcode": "2025GCN.39820....1A",
  "submittedHow": "email",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "circularId": 39820
}