TITLE: GCN CIRCULAR NUMBER: 30170 SUBJECT: GRB 210610B: Swift detection of a burst with bright optical afterglow DATE: 21/06/10 20:06:36 GMT FROM: David Palmer at LANL K. L. Page (U Leicester), J.D. Gropp (PSU), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 19:51:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210610B (trigger=1054681). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 243.939, +14.391 which is RA(J2000) = 16h 15m 45s Dec(J2000) = +14d 23' 29" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a comlex structure with a duration of about 100 sec. The peak count rate was ~11000 counts/sec (15-350 keV), at ~8 sec after the trigger. The XRT began observing the field at 19:52:51.4 UT, 83.9 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 243.9180, 14.3977 which is equivalent to: RA(J2000) = 16h 15m 40.32s Dec(J2000) = +14d 23' 51.7" with an uncertainty of 6.7 arcseconds (radius, 90% containment). This location is 77 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 0.1 s image was 1.07e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 91 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 16:15:40.41 = 243.91836 DEC(J2000) = +14:23:56.7 = 14.39909 with a 90%-confidence error radius of about 0.61 arc sec. This position is 5.2 arc sec. from the center of the XRT error circle. The estimated magnitude is 13.70 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.044. Burst Advocate for this burst is K. L. Page (klp5 AT leicester.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/)