TITLE:   GCN CIRCULAR
NUMBER:  7609
SUBJECT: GRB 080413a, SMARTS optical/IR afterglow observations
DATE:    08/04/13 19:22:38 GMT
FROM:    Bethany Cobb at Yale U  <cobb@astro.yale.edu>

B. E. Cobb (Yale), part of the larger SMARTS consortium, reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 080413a
(GCN 7594, Beardmore et al.), starting at 2008-04-13 05:48 UT,
which is ~2.9 hours post-burst. Several dithered images were obtained
in each filter, with total summed exposure times of 180s in each of
BRIYJK and 120s in each of H and V.

The GRB afterglow (GCN 7593 Rykoff & Rujopakarn; GCN 7594,
Beardmore et al.) is detected in all our images.  At
a mid-exposure time of 2008-04-13 06:02 (~3.1 hours post-burst)
the afterglow has the following magnitudes (which have not been
corrected for Galactic reddening):

B= 20.88 +/- 0.10
V= 20.25 +/- 0.11
R= 19.25 +/- 0.05
I= 18.84 +/- 0.06
J= 18.23 +/- 0.23
H= 17.05 +/- 0.15
K= 16.19 +/- 0.12

Optical photometry is calibrated against Landolt standard stars
and IR photometry is calibrated against 2MASS stars in the field.

Between 3.1 and 5.1 hours post burst, the afterglow decays
by ~0.75 magnitudes, indicating an approximate decay rate of alpha = -1.4
(where afterglow flux is proportional to t^alpha).