TITLE: GCN CIRCULAR NUMBER: 30652 SUBJECT: GRB 210818A: Swift detection of a burst with an optical counterpart DATE: 21/08/18 01:26:40 GMT FROM: Kim Page at U.of Leicester A. P. Beardmore (U Leicester), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 01:02:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210818A (trigger=1069169). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 149.131, -87.031 which is RA(J2000) = 09h 56m 32s Dec(J2000) = -87d 01' 52" 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 60 sec. The peak count rate was ~28000 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 01:03:06.9 UT, 59.0 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source with an enhanced position: RA, Dec 148.8666, -87.0253 which is equivalent to: RA(J2000) = 09h 55m 27.99s Dec(J2000) = -87d 01' 31.0" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 53 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 in excess of the Galactic value (1.74 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 4.7 (+2.38/-2.11) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 1.40e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 66 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 09:55:28.15 = 148.86729 DEC(J2000) = -87:01:28.8 = -87.02467 with a 90%-confidence error radius of about 0.62 arc sec. This position is 1.9 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.93 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.213. Although this was originally reported in the GCN Notices as a known source we believe that this is a GRB. Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk). 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/)