{
  "bibcode": "2020GCN.27696....1B",
  "body": "S. D. Barthelmy (GSFC), J.D. Gropp (PSU), J. A. Kennea (PSU),\nN. P. M. Kuin (UCL-MSSL), S. Laha (GSFC/UMBC/CRESST),\nF. E. Marshall (NASA/GSFC), M. J. Moss (GWU),\nK. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU),\nM. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of\nthe Neil Gehrels Swift Observatory Team:\n\nAt 17:36:50 UT, the Swift Burst Alert Telescope (BAT) triggered on\nSwift J1818.0-1607 (trigger=969823).  Swift slewed immediately to the source. \nThe BAT on-board calculated location is \nRA, Dec 274.498, -16.152, which is \n   RA(J2000)  =  18h 18m 00s\n   Dec(J2000) = -16d 09' 06\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve shows a single spike\nwith a duration of about 0.3 sec.  The peak count rate\nwas ~1200 counts/sec (15-350 keV), at ~0 sec after the trigger. \n\nThe XRT began observing the field at 17:37:52.5 UT, 62.6 seconds after\nthe BAT trigger. Using promptly downlinked data we find an \nX-ray source with an enhanced position: RA, Dec 274.5008, -16.1312\nwhich is equivalent to:\n   RA(J2000)  = 18h 18m 00.18s\n   Dec(J2000) = -16d 07' 52.3\"\nwith an uncertainty of 3.6 arcseconds (radius, 90% containment). This\nlocation is 75 arcseconds from the BAT onboard position, within the BAT\nerror circle, and consistent with the known position of Swift J1818.0-1607. \nThis position may be improved as more data are received;\nthe latest position is available at https://www.swift.ac.uk/sper.  We\ncannot determine whether the source is fading at the present time. \n\nA power-law fit to a spectrum formed from promptly downlinked event\ndata gives a column density consistent with the Galactic value of 1.35\nx 10^22 cm^-2 (Willingale et al. 2013). \n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 65 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 large, but uncertain, extinction expected. \n\nThis source was first reported on 2020 March 12 (GCN #27373; Evans et al.)\nSince then it has been monitored every 3-7 days with Swift follow-up \nobservations.",
  "circularId": 27696,
  "createdOn": 1588787755000,
  "email": "scott@milkyway.gsfc.nasa.gov",
  "subject": "Swift Detection of a burst from SGR Swift J1818.0-1607",
  "submitter": "Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>"
}