GCN Circular 11042

D. Xu (Weizmann Inst.), D. Malesani (DARK/NBI), P. Jakobsson (Univ. of
Iceland), P. A. Wilson, R. Nilsson (NOT), report on behalf of a larger
collaboration:

We have imaged the field of GRB100802A (Troja et al., GCN #11031) with
the Nordic Optical Telescope equipped with ALFOSC. Observations
started at 02:49 UT, 3rd August (~21.06 hrs after the BAT trigger) and
four 900 s frames in the R band were obtained.

We detect an optical source localised at

RA(J2000) = 00:09:52.38
Dec(J2000) = +47:45:18.8

with an error radius of 0.4". This position is 0.96" +/- 0.85" S-W of
the UVOT position (Siegel & Troja, GCN #11032). We therefore identify
this source as the afterglow of GRB100802A. A finding chart is visible
at the following URL:

http://www.astro.ku.dk/~malesani/GRB/100802A/GRB100802A_NOT.jpg.

Based on the USNO-B1 catalog (R1 magnitudes), we find R=22.9+/-0.3 at
a mean epoch of 21.6 hr after the BAT trigger. Compared with the P60
detection (Cenko, GCN #11040), our measurement confirms a clear
fading, and implies a rather shallow decay of the afterglow light
curve with alpha = 0.34 +/- 0.08 assuming an unbroken power-law decay
F(t) propto t^-alpha. This might suggest some contribution from an
underlying host galaxy.