GCN Circular 8075

T. Kruehler, F. Schrey, J. Greiner, A. Yoldas, C. Clemens, S. McBreen 
(all MPE Garching),
A. Kupcu Yoldas (ESO) and G. Szokoly (Eoetvoes Univ., Budapest) report on
behalf of the GROND team:

GROND (Greiner et al. 2008, PASP 120, 405), mounted at the 2.2m ESO/MPI
telescope at La Silla Observatory (Chile), started observations of the
field of GRB 080804 (Racusin et al. 2008, GCN #8057) in g'r'i'z'JHK at
23:26 UT, 16 min after the burst. We clearly detect the optical
afterglow (Rujopakarn & Rykoff 2008, GCN #8056) in all filters at a
position consistent with UVOT (Racusin et al. 2008, GCN #8057) and the
Faulkes Telescope South (Guidorzi & Steele, GCN #8064). Observations
continued for 65 min and were resumed at 06:30 UT for 30 min.

Assuming the USNO-B1 star at RA(J2000)=328.6627, DEC(J2000)=-53.18288
having R2=18.08 mag, we measure the following r' magnitudes, not
corrected for the expected Galactic foreground extinction corresponding
to a reddening of E_(B-V)=0.02 (Schlegel et al. 1998):

Filter T_mid    Exposure     Brightness
          (s)         (s)               (mag)
-----------------------------------------
r'      1160       67        18.09 +- 0.03
r'      1565       67        18.37 +- 0.03
r'      1996       115       18.66 +- 0.02
r'      2768       115       18.96 +- 0.02
r'      3672       375       19.25 +- 0.02
r'      26112     375       21.02 +- 0.04


The reported magnitudes are a subset of the complete observation
consisting of 24 individual frames. The total r band light curve is well
fitted by power law with index of 0.87 +- 0.03 between 0.32 h and 7.63 h
post burst. This decline is consistent with the Faulkes Telescope South
detection (Guidorzi & Steele, GCN #8064) at 14.235 h post burst.

Assuming the optical light curve continues the decline with the same 
power law index, we predict r' band magnitudes of 22.7 at 2 days and 
23.4 at 4 days post burst.

The broad band SED from g' to K can be fitted by a power law with index
0.8 +- 0.1 (statistical) +-0.1 (systematic), where the latter is due to 
the lack of photometric calibration.