[vsnet-grb-info 3631] GRB 060729, SMARTS optical/IR observations

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5465
SUBJECT: GRB 060729, SMARTS optical/IR observations
DATE:    06/08/18 15:15:20 GMT
FROM:    Bethany Cobb at Yale U  <cobb at astro.yale.edu>

B. E. Cobb and C. D. Bailyn, part of the larger SMARTS consortium, report:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 060729
(Grupe et al. GCN 5365) with the following mid-exposure times:
        obs1 2006-08-03 10:07 UT -  4.6 days post-burst
        obs2 2006-08-12 10:11 UT - 13.6 days post-burst
        obs3 2006-08-14 09:26 UT - 15.6 days post-burst
        obs4 2006-08-16 09:27 UT - 17.6 days post-burst
For each observation, total summed exposure times amounted to
36 minutes in I and 30 minutes in J.

The afterglow of GRB 060729 is clearly visible in the I-band in
our first observation (obs1), despite 4.6 days having past since the
GRB occurred.  The preliminary afterglow magnitude is I = 20.0 +/- 0.1.
The I-band photometry is calibrated to a number of secondary stars in the
field of GRB 060729.  The magnitude of these secondary stars was
derived using Landolt standard star observations obtained
during the night of obs4, which was a photometric night at CTIO.
The afterglow is only marginally detected in the corresponding
J-band image, which has an approximate limiting magnitude of J > 19
(calibrated based on several 2MASS stars in the field).

No source appears at the position of the afterglow in the individual
observations taken 13.6 to 17.6 days post-burst (obs2 - obs4). 
When these three images are combined, however, a faint source is 
detected in the I-band, with magnitude I = 21.9 +/- 0.2
(no source is detected in the combined J-band image to a limiting
magnitude of J > 19.5).  A decay of ~2 magnitudes between day ~5
and day ~16 is consistent with an afterglow decay rate
of alpha ~ -1.5, but steeper than the late-time optical
decay noted by Grupe (GCN 5432).  If the host galaxy contributes
significantly, the afterglow decay must be steeper still.