GCN Circular 6169

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.