GCN Circular 6425

A. Moretti (INAF-OAB), S. D. Barthelmy (GSFC), M. M. Chester (PSU),
N. Gehrels (NASA/GSFC), C. Gronwall (PSU), J. A. Kennea (PSU),
V. Mangano (INAF-IASFPA), P. T. O'Brien (U Leicester),
D. M. Palmer (LANL), P. Romano (Univ. Bicocca & INAF-OAB),
M. Stamatikos (NASA/ORAU), E. Troja (INAF-IASFPA) and
S. D. Vergani (DIAS-DCU) report on behalf of the Swift Team:

At 17:44:53 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070520B (trigger=279898).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 121.951, +57.617 which is 
   RA(J2000)  =  08h 07m 48s
   Dec(J2000) = +57d 37' 00"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 5 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. 
This burst occurred during a Malindi data downlink session
which buffered the initial notices for up to 12 minutes. 

The XRT began observing the field at 17:46:31 UT, 98 seconds after the
BAT trigger. Using prompt downlinked data 
XRT found a bright and flaring uncatalogued X-ray source
located at RA, Dec 121.8798, 57.6092 which is
   RA(J2000)  = 08h 07m 31.1's
   Dec(J2000) = +57d 36' 33.1''
with an uncertainty of 4.3 arcsec (radius, 90% containment). 
This location is 138 arcseconds from the BAT on-board position,
within the BAT error circle. The initial flux in the 2.5s image
was 1.5e-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 107 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 at the XRT position is 19.5 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 Galactic reddening of E(B-V)=0.04.