TITLE: GCN CIRCULAR NUMBER: 23349 SUBJECT: GRB 181020A: Swift detection of a burst with an optical counterpart DATE: 18/10/20 19:19:43 GMT FROM: David Palmer at LANL M. J. Moss (George Washington University), A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC), J.D. Gropp (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 19:00:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181020A (trigger=867987). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 13.967, -47.380 which is RA(J2000) = 00h 55m 52s Dec(J2000) = -47d 22' 47" 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 30 sec. The peak count rate was ~12000 counts/sec (15-350 keV), at ~7 sec after the trigger. The XRT began observing the field at 19:01:29.0 UT, 55.6 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 13.9837, -47.3796 which is equivalent to: RA(J2000) = 00h 55m 56.09s Dec(J2000) = -47d 22' 46.6" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 40 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. The initial flux in the 2.5 s image was 6.04e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 121 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 00:55:55.78 = 13.98240 DEC(J2000) = -47:22:51.8 = -47.38105 with a 90%-confidence error radius of about 0.76 arc sec. This position is 6.1 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.51 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. Burst Advocate for this burst is M. J. Moss (mikejmoss3 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/too.html.)