TITLE: GCN CIRCULAR NUMBER: 20283 SUBJECT: GRB 161218A: Swift detection of a burst DATE: 16/12/18 04:01:36 GMT FROM: David Palmer at LANL A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC), H. A. Krimm (CRESST/NSF/USRA), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 03:47:36 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 161218A (trigger=727288). Swift could not slew to the burst do to an observing constraint. The BAT on-board calculated location is RA, Dec 245.257, -4.114 which is RA(J2000) = 16h 21m 02s Dec(J2000) = -04d 06' 48" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single complex structure with a duration of about 10 sec. The peak count rate was ~20000 counts/sec (15-350 keV), at ~1 sec after the trigger. Due to a Sun observing constraint, Swift cannot slew to the BAT position until 15:54 UT on 2017 January 08. There will thus be no XRT or UVOT data for this trigger before this time. Burst Advocate for this burst is A. D'Ai (antonino.dai AT ifc.inaf.it). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.)