GCN Circular 6296

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 070318
(GCN 6210
Not found
 
 
, Cummings et al.) with a mid-exposure time of
2007-03-19 00:09 UT (~16.7 hrs post-burst) and again
at 2007-03-21 00:07 UT (~64.6 hrs post-burst).  For each
set of observations, total summed exposure times amounted
to 36 minutes in I and 30 minutes in J.

The afterglow of GRB 070318 (GRB 6210, Cummings et al.) is detected
in the first epoch images and is observed to fade between the
first and second epochs.

time-post
bursts (hrs)    I magnitude             J magnitude
------------------------------------------------------
16.7            19.82+/-0.07            18.29+/-0.11
64.6            20.28+/-0.12            > 18.6 (3 sigma limit)

Magnitudes are calibrated using Landolt standard stars in the
optical and 2MASS stars in the IR.

The optical decay rate (afterglow flux proportional to t^-alpha)
between 16.7 and 64.6 hours post-burst is alpha ~ 0.3 (and
the IR decay is consistent with this value).  This unusually
shallow decay was also noted in the UVOT observations
of this afterglow (GCN 6220
Not found
 
 
, Page et al.).

A third late-time epoch was obtained at 2007-04-06 23:46 UT
(~20 days post-burst). No source or host galaxy is detected
at the position of the optical afterglow to a limiting magnitude
of I > 21.7 and J > 19.2.  This indicates that the decay
rate of the afterglow must have steepened sometime after our
second epoch observation.