[vsnet-grb-info 6463] GRB 080804: GROND observations

[GCN Circulars]

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

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.