TITLE:   GCN CIRCULAR
NUMBER:  8547
SUBJECT: GRB 081121, SMARTS optical/IR afterglow observations
DATE:    08/11/23 00:27:49 GMT
FROM:    Bethany Cobb at UC Berkeley  <bcobb@astro.berkeley.edu>

B. E. Cobb (UC Berkeley) reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained several epochs of optical/IR imaging of the
error region of GRB 081121 (GCN 8537, Oates et al.).  Each
epoch consisted of several dithered images 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 2008-11-22 02:02 (5.4 hrs post-burst),
the GRB afterglow (GCN 8536, Yuan et al.) is detected with the
following magnitudes:
B = 19.34 +/- 0.04
R = 18.61 +/- 0.04
I = 18.21 +/- 0.04
J = 17.21 +/- 0.15
H = 17.05 +/- 0.15
K = 16.75 +/- 0.15

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 ~0.3 magnitudes.  In the IR, calibration is against 
2MASS stars.

Between 5.4 hrs and 8.7 hours post-burst, the afterglow decays
with an approximate optical decay rate of alpha~-1 (where
afterglow flux is proportional to t^alpha).