B. E. Cobb (UC Berkeley) reports:
Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 081008
(GCN 8344, Racusin et al.). Several dithered images were obtained
in each filter, with total summed exposure times of 180s in each
of BRIJK and 120s in each of H and V.
At a mid-exposure time of 2008-10-08 23:53 (3.9 hrs post-burst),
the GRB afterglow (GCN 8343, Rykoff et al. and GCN 8344, Racusin et al.)
is detected with the following magnitudes:
B = 19.86 +/- 0.07
R = 18.46 +/- 0.04
I = 18.09 +/- 0.05
J = 17.13 +/- 0.10
H = 16.52 +/- 0.08
K = 16.08 +/- 0.09
Observations were obtained under non-photometric conditions. In optical,
preliminary magnitudes are calibrated against several USNO-B1.0 stars in
the field, so there is likely an additional photometric calibration error
of ~0.2 magnitudes. In the IR, calibration is against 2MASS stars.
Additional observations were obtained at 5.0 hrs and 6.4 hrs post-burst.
Between 3.9 hrs and 6.4 hours post-burst, the afterglow decays
rapidly with an approximate optical decay rate of alpha~-1.5 (where
afterglow flux is proportional to t^alpha).
[GCN OPS NOTE(27oct08): Per author's request, the errors for the J,H,K
measurements from 0.97,0.77,0.90 to 0.10,0.08,0.09, respectively.]