TITLE:   GCN CIRCULAR
NUMBER:  6757
SUBJECT: GRB 070810A: Continued Keck Imaging
DATE:    07/08/12 05:08:12 GMT
FROM:    Daniel Perley at U.C. Berkeley  <dperley@astro.berkeley.edu>

D. A. Perley,  J. S. Bloom (UC Berkeley),  and C. C. Thoene (DARK, UCB)
report:

Beginning at 06:48:00 UT (2007-08-11) we began a second series of imaging
exposures on the field of GRB 070810A (GCN 7433) using the Keck I
telescope (+LRIS).  We acquired a total of 390s in R band and 480s in g
band through high airmass.

The optical afterglow (GCN 6734) is still detected, but has faded
significantly since our previous imaging epoch (GCN 6741) and appears
marginally extended.  Aperture photometry of the source indicates that it
has has faded by 3.2 +/- 0.1 magnitudes since the observations the first
night, corresponding to a decay index of alpha >= 1.5 between 4 hours and
28.6 hours including a possible contribution from a host galaxy or other
extended source.

Using the same calibration system as in our previous circular, we estimate
a preliminary magnitude of R=23.7.