GCN Circular 6423

P. Romano (Univ. Bicocca & INAF-OAB), S. D. Barthelmy (GSFC),
M. M. Chester (PSU), N. Gehrels (NASA/GSFC), C. Gronwall (PSU),
C. Guidorzi (Univ Bicocca&INAF-OAB), V. Mangano (INAF-IASFPA),
F. E. Marshall (NASA/GSFC), C. Pagani (PSU), D. M. Palmer (LANL),
M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester) and
S. D. Vergani (DIAS-DCU) report on behalf of the Swift Team:

At 13:05:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070520 (trigger=279817).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 193.316, +74.993 which is 
   RA(J2000)  =  12h 53m 16s
   Dec(J2000) = +74d 59' 36"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). This is an 88-sec image trigger, and as such
the TDRSS lightcurve does not show anything significant above background,
as is typical for image triggers. 

The XRT began observing the field at 13:07:48 UT, 159 seconds after the
BAT trigger. XRT found a  X-ray source
located at RA, Dec 193.3636, +74.9909 which is
   RA(J2000)  =  12h 53m 27.2s
   Dec(J2000) =  74d 59' 27.2"
with an uncertainty of 5.4 arcseconds (radius, 90% containment). 
This location is 45 arcseconds from the BAT on-board position,
within the BAT error circle. The initial flux in the 2.5s image
was 1.1e-09 erg/cm2/s (0.2-10 keV). 

UVOT took a finding chart exposure of  77 seconds with the White (160-650 nm)
filter starting 168 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 19.6 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
0.02.