[vsnet-grb-info 5332] GRB 071031: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7020
SUBJECT: GRB 071031: Swift detection of a burst with an optical counterpart
DATE:    07/10/31 01:24:38 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

M. C. Stroh (PSU), C. Gronwall (PSU), D. Grupe (PSU),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
H. A. Krimm (GSFC/USRA), C. Pagani (PSU), D. M. Palmer (LANL),
M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester),
T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 01:06:36 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 071031 (trigger=295670).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 6.458, -58.045 which is 
   RA(J2000) = 00h 25m 50s
   Dec(J2000) = -58d 02' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peaked
structure with a duration of about 30 sec.  The peak count rate
was ~600 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 01:08:19 UT, 103 seconds after the
BAT trigger. XRT found a bright, fading, uncatalogued X-ray source
located at RA, Dec 6.4061, -58.0585 which is
   RA(J2000)  =  00h 25m 37.4s
   Dec(J2000) = -58d 03' 30.6"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). 
This location is 110 arcseconds from the BAT on-board position,
within the BAT error circle. The initial flux in the 2.5s image
was 5.2e-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 112 seconds after the BAT trigger. There is a candidate afterglow
in the rapidly available 2.7'x2.7' sub-image at
   RA(J2000) =	00:25:37.27 =	6.4053
   DEC(J2000) = -58:03:34.2 = -58.0595
with a 1-sigma error radius of about 0.5 arc sec. This position is 3.9 arc sec. 
from the center of the XRT error circle. The estimated magnitude is 19.8 with a
1-sigma error of about 0.5 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is M. C. Stroh (stroh AT astroh.org). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)