TITLE: GCN CIRCULAR NUMBER: 26534 SUBJECT: GRB 191221B: Swift detection of a burst and a very bright optical candidate DATE: 19/12/21 21:00:24 GMT FROM: Sibasish Laha at GSFC S Laha (GSFC/UMBC/CRESST), S. D. Barthelmy (GSFC), J.D. Gropp (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and K. K. Simpson (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 20:39:13 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191221B (trigger=945521). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 154.832, -38.141 which is RA(J2000) = 10h 19m 20s Dec(J2000) = -38d 08' 28" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 15 sec. The peak count rate was ~17000 counts/sec (15-350 keV), at ~5 sec after the trigger. The XRT began observing the field at 20:40:39.3 UT, 86.2 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 154.8286, -38.1579 which is equivalent to: RA(J2000) = 10h 19m 18.86s Dec(J2000) = -38d 09' 28.4" with an uncertainty of 6.8 arcseconds (radius, 90% containment). This location is 61 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. UVOT took a finding chart exposure of 150 seconds with the White filter starting 91 seconds after the BAT trigger. A very bright afterglow candidate has been found at the XRT position in the initial data products. Because of coincidence loss we cannot calculate the magnitude accurately at this time. No correction has been made for the expected extinction corresponding to E(B-V) of 0.08. Burst Advocate for this burst is S Laha (sib.laha AT gmail.com). 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/)