TITLE: GCN CIRCULAR NUMBER: 7525 SUBJECT: GRB 080328: Swift detection of a burst with an optical counterpart DATE: 08/03/28 08:28:44 GMT FROM: David Palmer at LANL M. Perri (ASDC), D. N. Burrows (PSU), S. Campana (INAF-OAB), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Guidorzi (INAF-OAB), C. B. Markwardt (CRESST/GSFC/UMD), K. L. Page (U Leicester), D. M. Palmer (LANL), D. Perez (U Leicester), P. Romano (INAF-IASFPA), R. L. C. Starling (U Leicester), G. Stratta (ASDC), D. E. Vanden Berk (PSU), P. A. Ward (MSSL-UCL) and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team: At 08:03:04 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 080328 (trigger=307931). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 80.479, +47.524 which is RA(J2000) = 05h 21m 55s Dec(J2000) = +47d 31' 26" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 110 sec. The peak count rate was ~9500 counts/sec (15-350 keV), at ~10 sec after the trigger. The XRT began observing the field at 08:04:44.0 UT, 99.4 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 80.48189, 47.51035 which is equivalent to: RA(J2000) = 05h 21m 55.65s Dec(J2000) = +47d 30' 37.3" with an uncertainty of 2.6 arcseconds (radius, 90% containment). This location is 49 arcseconds from the BAT onboard position, within the BAT error circle. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 3.38e+21 cm^-2 (Kalberla et al. 2005), so we cannot constrain the redshift at this time using the relation from Grupe et al. (2007). A summary of the promptly downlinked data is given at http://www.swift.ac.uk/sper/307931/. The initial flux in the 2.5 s image was 1.08e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of nominal 400 seconds with the V filter starting 216 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 05:21:55.85 = 80.4827 DEC(J2000) = +47:30:39.2 = 47.5109 with a 1-sigma error radius of about 0.6 arc sec. This position is 2.8 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.7 with a 1-sigma error of about 0.5 mag. No correction has been made for the expected extinction of about 1.9 magnitudes. Burst Advocate for this burst is M. Perri (perri AT asdc.asi.it). 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.)