TITLE:   GCN CIRCULAR
NUMBER:  6466
SUBJECT: GRB 070529: Swift detection of a burst with an optical aterglow
DATE:    07/05/29 13:13:10 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

S. T. Holland (CRESST/GSFC/USRA), L. M. Barbier (NASA/GSFC),
S. D. Barthelmy (GSFC), P. J. Brown (PSU), S. Campana (INAF-OAB),
G. Cusumano (INAF-IASFPA), P. A. Evans (U Leicester),
N. Gehrels (NASA/GSFC), O. Godet (U Leicester), C. Gronwall (PSU),
C. Guidorzi (Univ Bicocca&INAF-OAB), H. A. Krimm (GSFC/USRA),
V. La Parola (INAF-IASFPA), V. Mangano (INAF-IASFPA),
F. E. Marshall (NASA/GSFC), J. P. Osborne (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
P. Romano (Univ. Bicocca & INAF-OAB), G. Sato (GSFC/ISAS),
B. Sbarufatti (INAF-IASFPA), R. L. C. Starling (U Leicester),
G. Tagliaferri (INAF-OAB), E. Troja (INAF-IASFPA),
D. E. Vanden Berk (PSU), S. D. Vergani (DIAS-DCU), L. Vetere (PSU)
and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team:

At 12:48:28 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070529 (trigger=280706).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 283.750, +20.639 which is 
   RA(J2000) = 18h 55m 00s
   Dec(J2000) = +20d 38' 21"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure at least 40 seconds long.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 12:50:39 UT, 131 seconds after the
BAT trigger. XRT found a moderately bright, uncatalogued X-ray source. 
From downlinked data, we determined the position RA, Dec 283.7427, +20.6586 
which is
   RA(J2000)  =  18h 54m 58.26s
   Dec(J2000) =  +20d 39' 31.04"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). 
This location is 76 arcseconds from the BAT on-board position,
within the BAT error circle. The initial flux in the 2.5s image
was 7.2e-10 erg/cm2/s (0.2-10 keV). 

UVOT took a finding chart exposure of 400 seconds with the V filter
starting 134 seconds after the BAT trigger. There is an afterglow
candidate with V=17.4 mag located at RA, Dec 283.7425, +20.6594 which
is     
   RA(J2000) = 18h 54m 58.19s     
   Dec(J2000) = 20d 39' 34.0"  with
an uncertainty of 0.5 arcsec (radius, 90% containment). The 2.7'x2.7'
sub-image covers 100% of the XRT error circle.   No correction has been
made for the expected extinction of about 1.0 magnitudes.