[vsnet-grb-info 6756] GRB 081008, SMARTS optical/IR afterglow observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8356
SUBJECT: GRB 081008, SMARTS optical/IR afterglow observations
DATE:    08/10/11 01:29:39 GMT
FROM:    Bethany Cobb at UC Berkeley  <bcobb at astro.berkeley.edu>

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 +/- 0.97
H = 16.52 +/- 0.77
K = 16.08 +/- 0.90

Observations were obtained under non-photometric conditions.  In optical, 
these
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).