TITLE: GCN CIRCULAR NUMBER: 30610 SUBJECT: GRB 210807C: Swift detection of a burst DATE: 21/08/07 23:14:05 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 22:57:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210807C (trigger=1064421). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 143.551, -51.409 which is RA(J2000) = 09h 34m 12s Dec(J2000) = -51d 24' 31" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Although the lightcurve before T+8s is not currently available due to a telemetry dropout, the later lightcurve shows a complex structure extending at least to T+90s. The peak count rate in the available lightcurve was ~800 counts/sec (15-350 keV), at ~20 sec after the trigger. The XRT began observing the field at 23:00:05.5 UT, 177.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 143.53455, -51.40237 which is equivalent to: RA(J2000) = 09h 34m 08.29s Dec(J2000) = -51d 24' 08.5" with an uncertainty of 7.1 arcseconds (radius, 90% containment). This location is 43 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data does not constrain the column density. The initial flux in the 2.5 s image was 2.31e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the V filter starting 180 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18.0 mag. No correction has been made for the large, but uncertain, extinction expected. 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/)