GCN Circular 3843
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.