TITLE: GCN CIRCULAR NUMBER: 8075 SUBJECT: GRB 080804: GROND observations DATE: 08/08/06 07:58:12 GMT FROM: Thomas Kruehler at MPE/MPI 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.