TITLE: GCN CIRCULAR NUMBER: 7988 SUBJECT: GRB 080721: Swift detection of a burst with a bright optical counterpart. DATE: 08/07/21 10:42:45 GMT FROM: David Palmer at LANL F. E. Marshall (NASA/GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Gronwall (PSU), C. Guidorzi (INAF-OAB), S. T. Holland (CRESST/USRA/GSFC), V. Mangano (INAF-IASFPA), P. T. O'Brien (U Leicester), J. P. Osborne (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), D. Perez (U Leicester), M. Perri (ASDC), B. Preger (ASDC), P. Romano (INAF-IASFPA), M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester), T. N. Ukwatta (GSFC/GWU), P. A. Ward (MSSL-UCL) and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team: At 10:25:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 080721 (trigger=317508). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 224.471, -11.712 which is RA(J2000) = 14h 57m 53s Dec(J2000) = -11d 42' 43" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed at least 4 peaks structure with a duration of about 35 sec. The peak count rate was ~19k counts/sec (15-350 keV), at ~0 sec after the trigger, during the second peak. The XRT began observing the field at 10:27:04.1 UT, 107.2 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 224.4899, -11.7242 which is equivalent to: RA(J2000) = 14h 57m 55.9s Dec(J2000) = -11d 43' 27.3" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 79 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. UVOT took a finding chart exposure of nominal 100 seconds with the White (160-650 nm) filter starting 180 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 14:57:55.8 = 224.48268 DEC(J2000) = -11:43:25 = -11.72361 with a 1-sigma error radius of about 1.0 arc sec. The estimated white magnitude is 14.6 with a 1-sigma error of about 0.01 mag. No correction has been made for Galactic extinction. Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.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/too.html.)