[vsnet-grb-info 4627] GRB 070508: Swift detection of an intense burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6383
SUBJECT: GRB 070508: Swift detection of an intense burst with an optical counterpart
DATE:    07/05/08 04:47:27 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at lheamail.gsfc.nasa.gov>

D. Grupe (PSU), S. D. Barthelmy (GSFC), M. M. Chester (PSU),
G. Cusumano (INAF-IASFPA), C. B. Markwardt (GSFC/UMD),
F. E. Marshall (NASA/GSFC), C. Pagani (PSU), D. M. Palmer (LANL),
J. L. Racusin (PSU), M. Stamatikos (NASA/ORAU) and L. Vetere (PSU)
report on behalf of the Swift Team:

At 04:18:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070508 (trigger=278854).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 312.783, -78.368 which is 
   RA(J2000)  =  20h 51m 08s
   Dec(J2000) = -78d 22' 04"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a complex multi-peaked
structure with a duration of at least 40 sec.  The peak count rate
was ~45000 counts/sec (15-350 keV), at ~15 sec after the trigger. 

The XRT began observing the field at 04:19:33 UT, 76 seconds after the
BAT trigger. XRT found a bright, uncatalogued X-ray source
located at RA, Dec 312.7959, -78.3850 which is
RA(J2000)  =  20h 51m 11.0s
Dec(J2000) = -78d 23' 06.0"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). 
This location is 62 arcseconds from the BAT on-board position,
within the BAT error circle. The initial flux in the 2.5s image
was 1.6e-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 85 seconds after the BAT trigger. There is a candidate
afterglow in the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	20:51:11.81 = 312.7992
  DEC(J2000) = -78:23:04.9  = -78.3847
with a 1-sigma error radius of about 0.5 arc sec. This position is 2.6 arc sec. 
from the center of the XRT error circle. The estimated magnitude is 19.4 with a
1-sigma error of about 0.5 mag. No correction has been made for the expected
reddening of E(B-V) of 0.14.