TITLE: GCN CIRCULAR NUMBER: 23482 SUBJECT: GRB 181202A: Swift detection of a burst with an optical counterpart DATE: 18/12/02 06:48:16 GMT FROM: David Palmer at LANL J. K. Cannizzo (NASA/UMBC), J.D. Gropp (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 06:36:24 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181202A (trigger=874334). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 280.745, +27.951 which is RA(J2000) = 18h 42m 59s Dec(J2000) = +27d 57' 03" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 10 sec. The peak count rate was ~1100 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 06:37:49.8 UT, 85.4 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 280.73563, 27.95894 which is equivalent to: RA(J2000) = 18h 42m 56.55s Dec(J2000) = +27d 57' 32.2" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 41 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 http://www.swift.ac.uk/sper. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.67 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 250 seconds with the U filter starting 86 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 18:42:56.38 = 280.73490 DEC(J2000) = +27:57:35.0 = 27.95973 with a 90%-confidence error radius of about 0.63 arc sec. This position is 3.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.67 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.14. Burst Advocate for this burst is J. K. Cannizzo (cannizzo AT milkyway.gsfc.nasa.gov). 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.)