GCN Circular 6475

C. B. Markwardt (GSFC/UMD), L. M. Barbier (NASA/GSFC),
S. D. Barthelmy (GSFC), M. M. Chester (PSU), N. Gehrels (NASA/GSFC),
S. D. Hunsberger (PSU), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL) and
M. Stamatikos (NASA/ORAU) report on behalf of the Swift Team:

At 02:10:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070531 (trigger=280958).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 6.678, +74.304 which is 
   RA(J2000)  =  00h 26m 43s
   Dec(J2000) = +74d 18' 14"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single FRED-like peak
with a duration of about 20 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began taking data at 02:12:25 UT, 128 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.  We are waiting for down-linked
data to detect and determine a position for the source. 

UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm)
filter starting 131 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
0.38.