[vsnet-grb-info 3276] GRB 060428a, SMARTS optical/IR afterglow detection

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5111
SUBJECT: GRB 060428a, SMARTS optical/IR afterglow detection
DATE:    06/05/10 23:56:25 GMT
FROM:    Bethany Cobb at Yale U  <cobb at 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 060428a
(GCN 5014, Mangano et al.) with a mid-exposure time of
2006-04-29 00:17 UT, which is ~20.9 hours post-burst and
again at 2006-04-30 23:46 UT (~68.4 hours post-burst).
For each observation total summed exposure times amounted
to 36 minutes in I and 30 minutes in J.

In our first epoch images, a source is visible in both I and J
within the X-ray error region reported by Mangano et al. (GCN 5018).
This source may correspond to the V-band source detected by
De Pasquale & Mangano (GCN 5024) in the Swift UVOT imaging.
The coordinates are:
RA: 8:14:10.8
DEC: -37:10:11.4

The source is absent in our second epoch images.
Its transient nature (see magnitudes below)
suggests that the source is the afterglow of GRB 060428a.

time
post-burst      I magnitude             J magnitude
------------------------------------------------------
20.9 hours      21.14 +/- 0.16          18.98 +/- 0.14
68.4 hours      >22.2+/-0.2             >19.6+/-0.2

These preliminary magnitudes were calibrated using several USNO-B1.0
stars in the I-band and several 2MASS standards in J.

The afterglow decay index is constrained by our observations to be
alpha >~ -0.8 from ~1 to 3 days post-burst. Because no optical
source was detected with magnitude <20 at only minutes post-burst
(see GCN 5033, Haislip et al. [z'>19.1 at 5.3 minutes] and GCN 5024, De 
Pasquale & Mangano [V=20 at 1 minute]), our I-band detection at 20.9 hours 
suggests that the early time decay rate of the afterglow must have been 
significantly more shallow than -0.8.  This may partially account for 
the lack of detected variability of the UVOT source at early times. 
Possibly, a light curve break occurred between our two sets of observations.