TITLE: GCN CIRCULAR NUMBER: 30600 SUBJECT: GRB 210807A: Swift detection of a burst with an optical counterpart DATE: 21/08/07 10:34:25 GMT FROM: David Palmer at LANL A. Y. Lien (GSFC/UMBC), C. Gronwall (PSU), J.D. Gropp (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII), K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 10:03:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210807A (trigger=1064221). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 76.677, +58.245 which is RA(J2000) = 05h 06m 43s Dec(J2000) = +58d 14' 43" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Because this is an image trigger, the actual light curve structure requires further ground data analysis. The 112-s image trigger indicates that this is a long burst. The XRT began observing the field at 10:07:02.3 UT, 201.9 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 76.6815, 58.2503 which is equivalent to: RA(J2000) = 05h 06m 43.56s Dec(J2000) = +58d 15' 01.1" with an uncertainty of 11.1 arcseconds (radius, 90% containment). This location is 20 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 4.08e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 210 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 05:06:43.90 = 76.68293 DEC(J2000) = +58:14:59.2 = 58.24978 with a 90%-confidence error radius of about 0.61 arc sec. This position is 3.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.00 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.409. 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/)