{
  "bibcode": "2021GCN.29597....1D",
  "body": "V. D'Elia (SSDC), E. Ambrosi (INAF-IASFPA), S. D. Barthelmy (GSFC),\nA. D'Ai (INAF-IASFPA), J. A. Kennea (PSU), N. J. Klingler (PSU),\nS. Laha (GSFC/UMBC/CRESST), F. E. Marshall (NASA/GSFC),\nD. M. Palmer (LANL) and A. Tohuvavohu (U Toronto) report on behalf of\nthe Neil Gehrels Swift Observatory Team:\n\nAt 03:53:57 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 210306A (trigger=1035994).  Swift slewed immediately to the burst. \nThe BAT on-board calculated location is \nRA, Dec 129.965, +60.196 which is \n   RA(J2000)  =  08h 39m 52s\n   Dec(J2000) = +60d 11' 46\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve shows multiple peaks in a complex\nstructure with a duration of about 15 sec.  The peak count rate\nwas ~20,000 counts/sec (15-350 keV), at ~7 sec after the trigger. \n\nThe XRT began observing the field at 03:55:12.07 UT, 74.9 seconds after\nthe BAT trigger. Using promptly downlinked data we find an\nuncatalogued X-ray source with a position: RA, Dec 129.96204, \n60.203271 which is equivalent to:\n   RA(J2000)  = 08h 39m 50.89s\n   Dec(J2000) = +60d 12' 11.7\"\nwith an uncertainty of 4.7 arcseconds (radius, 90% containment). This\nlocation is 27 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. \n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 85 seconds after the BAT trigger. There is a candidate afterglow in\nthe rapidly available 2.7'x2.7' sub-image at\n  RA(J2000)  =\t08:39:49.95 = 129.95813\n  DEC(J2000) = +60:12:19.4  =  60.20538\nwith a 90%-confidence error radius of about 0.61 arc sec. This position is 13.6\narc sec. from the center of the XRT error circle. The estimated magnitude is\n16.22 with a 1-sigma error of about  0.14. No correction has been made for the\nexpected extinction corresponding to E(B-V) of 0.052. \n\nBurst Advocate for this burst is V. D'Elia (delia AT ssdc.asi.it). \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/)",
  "circularId": 29597,
  "createdOn": 1615003948000,
  "email": "scott@milkyway.gsfc.nasa.gov",
  "subject": "GRB 210306A: Swift detection of a burst with a bright optical counterpart",
  "submitter": "Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>",
  "eventId": "GRB 210306A"
}