TITLE: GCN CIRCULAR NUMBER: 29192 SUBJECT: GRB 201229A: Swift detection of a burst with a possible optical counterpart DATE: 20/12/29 11:23:17 GMT FROM: Scott Barthelmy at NASA/GSFC B. Sbarufatti (PSU), S. D. Barthelmy (GSFC), J.D. Gropp (PSU) and A. Y. Lien (GSFC/UMBC) report on behalf of the Neil Gehrels Swift Observatory Team: At 10:59:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201229A (trigger=1015088). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 210.708, +48.197 which is RA(J2000) = 14h 02m 50s Dec(J2000) = +48d 11' 50" 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 7 sec. The peak count rate was ~500 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 11:01:22.2 UT, 111.4 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 210.6891, 48.1976 which is equivalent to: RA(J2000) = 14h 02m 45.39s Dec(J2000) = +48d 11' 51.2" with an uncertainty of 2.2 arcseconds (radius, 90% containment). This location is 45 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 1.78 x 10^20 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 116 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 14:02:45.41 = 210.68921 DEC(J2000) = +48:11:53.8 = 48.19828 with a 90%-confidence error radius of about 0.62 arc sec. This position is 3.1 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.19 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. Burst Advocate for this burst is B. Sbarufatti (bxs60 AT psu.edu). 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/)