TITLE: GCN CIRCULAR NUMBER: 8292 SUBJECT: GRB 080928: Swift detection of a burst with an optical counterpart DATE: 08/09/28 15:16:04 GMT FROM: Scott Barthelmy at NASA/GSFC T. Sakamoto (NASA/UMBC), S. D. Barthelmy (GSFC), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Gronwall (PSU), C. Guidorzi (INAF-OAB), S. T. Holland (CRESST/USRA/GSFC), J. Mao (INAF-OAB), F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), A. M. Parsons (GSFC) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 15:01:32 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 080928 (trigger=326115). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 95.084, -55.173 which is RA(J2000) = 06h 20m 20s Dec(J2000) = -55d 10' 23" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). This is an image trigger so as usual the BAT lightcurve shows nothing. The XRT began observing the field at 15:04:22.6 UT, 169.7 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 95.0691, -55.1997 which is equivalent to: RA(J2000) = 06h 20m 16.58s Dec(J2000) = -55d 11' 58.9" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 100 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 5.77e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 179 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 06:20:16.85 = 95.0702 DEC(J2000) = -55:11:59.3 = -55.1998 with a 1-sigma error radius of about 0.5 arc sec. This position is 2.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.2 with a 1-sigma error of about 0.5 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.07. Burst Advocate for this burst is T. Sakamoto (Taka.Sakamoto 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/too.html.)