[vsnet-grb-info 6472] GRB 080810: ROTSE-III Prompt Detection of Optical Counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8084
SUBJECT: GRB 080810: ROTSE-III Prompt Detection of Optical Counterpart 
DATE:    08/08/10 18:29:37 GMT
FROM:    Eli Rykoff at UCSB  <erykoff at physics.ucsb.edu>

E.S. Rykoff (UCSB), reports on behalf of the ROTSE collaboration:

ROTSE-IIIa, located at Siding Spring Observatory, Australia, responded
to GRB 080810 (Swift trigger 319584, GCN 8080, Page et al.). The first
image was at 13:10:47.6 UT, 35.3 s after the burst (21.6 s after the
GCN notice time), contemporaneous with the gamma-ray emission (GCN
8082, Sakamoto et al.).  We took 10 5s exposures, 10 20s exposures,
and 137 60s exposures under variable conditions.  The unfiltered
images are calibrated relative to USNO B1.0. We detect a 12.8
magnitude, variable source with coordinates:

   23:47:10.5 +00:19:11.5 (J2000) (uncertainty of 1" or better)

start UT        mag     mlim(of image)
----------------------------------

13:11:16.7     12.8     16.5

The position is consistent with the XRT and UVOT positions reported in
GCN 8080.

Our prompt detection shows the afterglow initially rising from 13.7
mag (38s post-burst) to a peak of 12.8 mag (67s post-burst).  This is
followed by a shallow decay from ~67s to ~230s (decay index
alpha=-0.15+/-0.05), and a break to a steeper decay from ~230s to
~6000s post-burst (decay index alpha=-1.13+/-0.02).  Preliminary
analysis does not show any significant deviations from this smooth
evolution.

A jpeg image is available at
http://www.rotse.net/images/gsb319584_3a02_img.jpg
The object in question is marked with yellow and blue circles.