TITLE: GCN CIRCULAR NUMBER: 6224 SUBJECT: GRB 070328: Swift detection of a burst DATE: 07/03/28 04:14:41 GMT FROM: David Palmer at LANL C. B. Markwardt (GSFC/UMD), M. M. Chester (PSU), J. R. Cummings (NASA/UMBC), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA), F. E. Marshall (NASA/GSFC), C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL), M. Stamatikos (NASA/ORAU) and E. Troja (INAF-IASFPA) report on behalf of the Swift Team: At 03:53:53 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070328 (trigger=272773). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 65.117, -34.069 which is RA(J2000) = 04h 20m 28s Dec(J2000) = -34d 04' 07" 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 70 sec. The peak count rate was ~8000 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 03:55:21 UT, 88 seconds after the BAT trigger. XRT found a bright, fading X-ray source located at RA, Dec 65.1160, -34.0646 which is RA(J2000) = 04h 20m 27.8s Dec(J2000) = -34d 03' 52.5" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 16 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 2.6e-09 erg/cm2/s (0.2-10 keV). UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 98 seconds after the BAT trigger. No 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 18.5 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 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.04.