TITLE: GCN CIRCULAR NUMBER: 30351 SUBJECT: GRB 210702A: Swift detection of a burst with a bright optical counterpart DATE: 21/07/02 19:31:14 GMT FROM: David Palmer at LANL A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), J. A. Kennea (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII), S. Laha (GSFC/UMBC/CRESST), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 19:07:13 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210702A (trigger=1058804). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 168.570, -36.741 which is RA(J2000) = 11h 14m 17s Dec(J2000) = -36d 44' 28" 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 100 sec. The peak count rate was ~10,000 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 19:08:49.2 UT, 95.5 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 168.5813, -36.7474 which is equivalent to: RA(J2000) = 11h 14m 19.51s Dec(J2000) = -36d 44' 50.6" with an uncertainty of 11.0 arcseconds (radius, 90% containment). 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 1.00e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 45 seconds with the U filter starting 315 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 11:14:18.83 = 168.57846 DEC(J2000) = -36:44:48.8 = -36.74690 with a 90%-confidence error radius of about 0.61 arc sec. This position is 8.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 11.72 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.121. Burst Advocate for this burst is A. Y. Lien (amy.y.lien AT 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/)