GCN Circular 10123

R. Filgas, F. Olivares, T. Kruehler, J. Greiner, (all MPE Garching)
report on behalf of the GROND team:

We observed the field of GRB 091102 (Swift trigger 374598; Hoversten
et al., GCN #10117) simultaneously in g'r'i'z'JHK with GROND (Greiner
et al. 2008, PASP, 120, 405) mounted at the 2.2 m ESO/MPI telescope at
La Silla Observatory (Chile). Observations started on November 3 at
00:02 UT, 9.5 hours after the GRB trigger. They were performed from
the beginning of the astronomical twilight at an average seeing of 2"
and at an average airmass of 2.15.

Although the source at the South-Western edge of the 2.2" Swift/XRT
error circle (Kennea et al., GCN #10120) reported by De Ugarte Postigo
et al. (GCN #10122) is clearly seen, we do not detect the afterglow
candidate in co-added images of 49.2 min integration time in g'r'i'z'
and 40 min in JHK down to the following limiting magnitudes (all in
the AB system):

r' > 23.1 mag,
i' > 22.9 mag,
z' > 23.1 mag,
J > 20.6 mag,
H > 20.5 mag, and
K > 19.0 mag,

calibrated against GROND zeropoints and 2MASS field stars. No
corrections for the expected Galactic foreground extinction were made,
which correspond to a reddening of E(B-V)=0.14 mag in the direction of
the burst (Schlegel et al. 1998).