GCN Circular 6431

C. Guidorzi (Univ Bicocca&INAF-OAB), S. D. Barthelmy (GSFC),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Gronwall (PSU),
C. B. Markwardt (GSFC/UMD), A. Moretti (INAF-OAB),
P. T. O'Brien (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), P. Romano (Univ. Bicocca & INAF-OAB) and
G. Tagliaferri (INAF-OAB) report on behalf of the Swift Team:

At 06:51:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070521 (trigger=279935).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 242.669, +30.241 which is 
   RA(J2000)  =  16h 10m 41s
   Dec(J2000) = +30d 14' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple peaks
with a duration of about 40 sec.  The peak count rate
was ~7000 counts/sec (15-350 keV), at ~20 sec after the trigger. 

The XRT began taking data at 06:52:27 UT, 77 seconds after the BAT
trigger.  The XRT on-board centroid algorithm did not find a source in the
image and no prompt position is available.  However, the downlinked X-ray
spectrum and lightcurve show that there is an X-ray object in the
field that appears to be fading. 
Using prompt downlinked data, we find a position RA, Dec 242.6606, 30.2579 which is
   RA(J2000)  = 16h10m 38.5s
   Dec(J2000) = +30d 15' 28.3''
with an uncertainty of 4.0 arcsec (radius, 90% containment). 


UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm)
filter starting 81 seconds after the BAT trigger. No afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of
the BAT 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
BAT 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.