{
  "bibcode": "2018GCN.23149....1M",
  "body": "F. E. Marshall (NASA/GSFC), V. D'Elia (ASDC),\nS. W. K Emery (UCL-MSSL), J. A. Kennea (PSU),\nM. J. Moss (George Washington University), K. L. Page (U Leicester)\nand M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift\nObservatory Team:\n\nAt 12:30:18 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 180818B (trigger=853882).  Swift slewed immediately to the burst. \nThe BAT on-board calculated location is \nRA, Dec 104.235, +39.329 which is \n   RA(J2000) = 06h 56m 56s\n   Dec(J2000) = +39d 19' 43\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve showed a complex\nstructure with a duration of about 130 sec, starting ~100 sec before\nthe trigger time during the previous spacecraft slew.  The peak count \nrate was ~1000 counts/sec (15-350 keV), at ~-90 sec after the trigger. \n\nThe XRT began observing the field at 12:33:42.4 UT, 203.8 seconds after\nthe BAT trigger. Using promptly downlinked data we find a bright,\nuncatalogued X-ray source located at RA, Dec 104.22468, 39.31662 which\nis equivalent to:\n   RA(J2000)  = 06h 56m 53.92s\n   Dec(J2000) = +39d 18' 59.8\"\nwith an uncertainty of 4.4 arcseconds (radius, 90% containment). This\nlocation is 53 arcseconds from the BAT onboard position, within the BAT\nerror circle. This position may be improved as more data are received;\nthe latest position is available at http://www.swift.ac.uk/sper.  We\ncannot determine whether the source is fading at the present time. No\nspectrum from the promptly downlinked event data is yet available to\ndetermine the column density. \n\nThe initial flux in the 2.5 s image was 1.30e-09 erg cm^-2 s^-1 (0.2-10\nkeV). \n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 212 seconds after the BAT trigger. No credible afterglow candidate has\nbeen found in the initial data products. The 2.7'x2.7' sub-image covers 100% of\nthe XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. \nThe 8'x8' region for the list of sources generated on-board covers 100% of the\nXRT error circle. The list of sources is typically complete to about 18 mag. No\ncorrection has been made for the expected extinction corresponding to E(B-V) of\n0.09. \n\nBurst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.nasa.gov). \nPlease contact the BA by email if you require additional information\nregarding Swift followup of this burst. In extremely urgent cases, after\ntrying the Burst Advocate, you can contact the Swift PI by phone (see\nSwift TOO web site for information: http://www.swift.psu.edu/too.html.)",
  "circularId": 23149,
  "createdOn": 1534596299000,
  "email": "palmer@lanl.gov",
  "subject": "GRB 180818B: Swift detection of a burst",
  "submitter": "David Palmer at LANL  <palmer@lanl.gov>",
  "eventId": "GRB 180818B"
}