{
  "bibcode": "2021GCN.30915....1E",
  "body": "P. A. Evans and K.L. Page (U. Leicester) report on behalf of the Swift-XRT team:\n\nGCN Circ (30914) was sent in error, prematurely. The corrected information is below,\nand we apologise for the confusion.\n\nWe have analysed 5.3 ks of XRT data for the Fermi-detected burst GRB 210928A\n(GCN Circs. 30912, 30913). The data were originally collected as a ToO to\nobserve U Sco, the possible counterpart to the Fermi-LAT source Fermi J1623-1752\n(ATEL #14939). While U Sco was not detected, a new transient, Swift\nJ162315.0-175233 was discovered (ATEL #14945). Further Swift observations were\nrequested and are ongoing. The discovery of an unannounced Fermi-GBM trigger has\nidentified this object as GRB 210928A (ATEL #14948), and the manual insertion \nof the already-observed data into the GRB analysis system resulted in the premature\ndispatch of this circular.\n\nThe Swift-XRT observations at present extend from 67.4 ks to 262.9 ks after the\nGBM trigger time. The data are entirely in Photon Counting (PC) mode. Using 1636\ns of PC mode data and 2 UVOT images, we find an enhanced XRT position (using the\nXRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue):\nRA, Dec = 245.51109, -17.94788 which is equivalent to:\n\nRA (J2000): 16h 22m 02.66s\nDec(J2000): -17d 56' 52.4\"\n\nwith an uncertainty of 4.4 arcsec (radius, 90% confidence). This\nposition is 29 arcsec from the Fermi-LAT  position. \n\nThe light curve can be modelled with a power-law decay with a decay index of\nalpha=1.3 (+1.8, -1.3); however comprising only 3 data points and an upper\nlimit, this should be viewed with the caution the large error-bars imply.\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 (+0.6, -0.3). The\nbest-fitting absorption column is  2.342 (+3.358, -0.021) x 10^21\ncm^-2, consistent with the Galactic value of 2.3 x 10^21 cm^-2\n(Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10\nkeV flux conversion factor deduced from this spectrum  is 4.8 x 10^-11\n(5.5 x 10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     2.342 (+3.358, -0.021) x 10^21 cm^-2\nGalactic foreground: 2.3 x 10^21 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.4 (+0.6, -0.3)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00014850.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 30915,
  "createdOn": 1633110533000,
  "email": "pae9@leicester.ac.uk",
  "subject": "GRB 210928A: Swift-XRT analysis (replaces GCN 30914)",
  "submitter": "Phil Evans at U of Leicester  <pae9@leicester.ac.uk>",
  "eventId": "GRB 210928A"
}