{
  "bibcode": "2018GCN.22462....1D",
  "body": "P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), A. Tohuvavohu (PSU), S. J.\nLaPorte (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester),\nA.P. Beardmore (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA)\nand P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the\nFermi/LAT-detected burst GRB 180305A (Axelsson et al. GCN Circ. 22457),\ncollecting 4.7 ks of Photon Counting (PC) mode data between T0+30.6 ks\nand T0+53.0 ks. \n\nFour uncatalogued X-ray sources are detected, of which one (\"Source 2\")\nis above the RASS limit and fading with 3-sigma significance, and is\ntherefore likely the GRB afterglow. Using 3574 s of PC mode data and 6\nUVOT images, we find an enhanced XRT position (using the XRT-UVOT\nalignment and matching UVOT field sources to the USNO-B1 catalogue):\nRA, Dec = 49.61821, +32.10994 which is equivalent to:\n\nRA (J2000): 03h 18m 28.37s\nDec(J2000): +32d 06' 35.8\"\n\nwith an uncertainty of 1.7 arcsec (radius, 90% confidence). This\nposition is 2.7 arcmin from the Fermi/LAT position. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=2.7 (+0.5, -0.8).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.5 (+/-0.4). The\nbest-fitting absorption column is  4.7 (+3.2, -2.3) x 10^21 cm^-2,\nconsistent with the Galactic value of 2.4 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 (6.9 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     4.7 (+3.2, -2.3) x 10^21 cm^-2\nGalactic foreground: 2.4 x 10^21 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.5 (+/-0.4)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00020789.\nThe results of the full analysis of the XRT observations are available\nat http://www.swift.ac.uk/ToO_GRBs/00020789.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 22462,
  "createdOn": 1520350722000,
  "email": "bxs60@psu.edu",
  "subject": "GRB 180305A: Swift-XRT afterglow detection",
  "submitter": "Boris Sbarufatti at PSU  <bxs60@psu.edu>",
  "eventId": "GRB 180305A"
}