TITLE: GCN CIRCULAR NUMBER: 26516 SUBJECT: GRB 191220A: Swift detection of a burst DATE: 19/12/20 14:13:01 GMT FROM: Kim Page at U.of Leicester S. Laha (GSFC/UMBC/CRESST), D. N. Burrows (PSU), J.D. Gropp (PSU), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 13:29:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191220A (trigger=945344). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 281.391, +26.667 which is RA(J2000) = 18h 45m 34s Dec(J2000) = +26d 40' 02" 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 20 sec. The peak count rate was ~200 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 13:31:50.2 UT, 132.4 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 281.37924, 26.65599 which is equivalent to: RA(J2000) = 18h 45m 31.02s Dec(J2000) = +26d 39' 21.6" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 54 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.53 x 10^21 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 4.03e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 137 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The 8'x8' region for the list of sources generated on-board covers 0.00% of the XRT error circle. No correction has been made for the expected extinction corresponding to E(B-V) of 0.14. 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/)