TITLE: GCN CIRCULAR NUMBER: 7753 SUBJECT: GRB 080520: Swift detection of a burst with optical afterglow DATE: 08/05/20 22:49:07 GMT FROM: David Palmer at LANL D. Grupe (PSU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), J. R. Cummings (NASA/UMBC), C. Gronwall (PSU), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), V. Mangano (INAF-IASFPA), K. M. McLean (GSFC/UMD), J. P. Osborne (U Leicester), D. M. Palmer (LANL), D. Perez (U Leicester), T. Sakamoto (NASA/UMBC), M. Stamatikos (NASA/ORAU), M. C. Stroh (PSU) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 22:20:24 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 080520 (trigger=312047). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 280.192, -55.004 which is RA(J2000) = 18h 40m 46s Dec(J2000) = -55d 00' 13" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows two peaks with a duration of about 15 sec. The peak count rate was ~700 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 22:22:04.3 UT, 99.5 seconds after the BAT trigger. Using promptly downlinked data we find a bright uncatalogued X-ray source. The position, astrometrically enhanced by aligning promptly available UVOT field source positions to the USNO-B1 catalogue, is RA,Dec = 280.19337, -54.99181 (degrees) which is equivalent to: RA (J2000.0) = 18 40 46.41 DEC (J2000.0) = -54 59 30.5 with an uncertainty of 2.8 arcsec (radius, 90% containment). This position is 44.0 arcsec from the BAT position, inside the BAT error circle. UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 106 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA (J2000.0) = 18:40:46.41 Dec (J2000.0) = -54:59:31.1 with an estimated uncertainty of 0.6 arcsec (90%, radius). The estimated magnitude is white = 19.8 +/- 0.1 mag. No correction has been made for Galactic extinction. Burst Advocate for this burst is D. Grupe (grupe AT astro.psu.edu). 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.)