TITLE: GCN CIRCULAR NUMBER: 6169 SUBJECT: GRB 070306: Swift detection of a burst DATE: 07/03/06 17:04:20 GMT FROM: Scott Barthelmy at NASA/GSFC S. B. Pandey (UCL-MSSL), S. D. Barthelmy (GSFC), J. R. Cummings (NASA/UMBC), M. De Pasquale (UCL-MSSL), N. Gehrels (NASA/GSFC), O. Godet (U Leicester), S. T. Holland (CRESST/GSFC/USRA), S. Immler (GSFC/USRA), J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA), V. Mangano (INAF-IASFPA), C. B. Markwardt (GSFC/UMD), F. E. Marshall (NASA/GSFC), A. Moretti (INAF-OAB), C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL), A. M. Parsons (GSFC), T. Sakamoto (NASA/ORAU), B. Sbarufatti (INAF-IASFPA), M. Stamatikos (NASA/ORAU), E. Troja (INAF-IASFPA) and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team: At 16:41:28 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070306 (trigger=263361). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 148.082, +10.471 which is RA(J2000) = 09h 52m 20s Dec(J2000) = +10d 28' 17" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). We did not receive a BAT lightcurve down TDRSS so we can not say anything about its shape. However, this 12 second trigger does have a strong image significance, and as such, the reliability is high. The XRT began observing the field at 16:44:01 UT, 153 seconds after the BAT trigger. XRT found a bright, fading and uncatalogued X-ray source located at RA, Dec 148.0980, +10.4827 which is RA(J2000) = 09h 52m 23.5s Dec(J2000) = 10d 28' 57.7" with an uncertainty of 6.1 arcseconds (radius, 90% containment). This location is 71 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 0.1s image was 1.3e-08 erg/cm2/s (0.2-10 keV). UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 162 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 3-sigma upper limit is about 19.8 mag. The 8'x8' region for the list of sources generated on-board also 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 0.03.