TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3843
SUBJECT: GRB 050820a: NIR Observations by WIRO
DATE:    05/08/21 05:51:50 GMT
FROM:    Daryl Macomb at Boise STate U  <dmacomb@boisestate.edu>

Daryl Macomb, Jessica Elias (Boise State U.), 
Jerry Bonnell, Alexander Kutyrev, Jay Norris (NASA/GSFC),
and Ron Canterna, Mike Pierce (U. Wyoming) report:

We observed the position reported by Fox and Cenko (GCN 3829)
of the afterglow for GRB 050820a (Swift trigger #151207, Page et al.,
GCN 3830) with the 2.3-meter WIRO telescope.  Starting at 08:10 UT 
(96 minutes after the GRB trigger) and continuing until 11:30 UT, 
we detected the fading afterglow in the J band in each of 120 40-s
integrations.  J magnitudes determined at the following times were
(relative to burst trigger):

Epoch   T-Ttrig     J mag
  1      6240 s      16.0
  2      9600 s      16.7
  3     16020 s      16.9

Between epochs 1 and 2, the inferred average power-law decay index 
is -1.5, steeper than the -0.9 index reported by Cenko and Fox
(GCN 3834) for the initial decay during the interval ~ 7 - 93 minutes 
after the burst.  Between epoches 2 and 3, the average decay index 
flattens to -0.44, after which our observations ended due to twilight.

The times of the three reported epochs are arbitrary.  Detailed 
analysis to yield accurate break times and indices is in progress.