S. Loew, J. Greiner, A. Yoldas (all MPE Garching), A. Kuepcue Yoldas (ESO),
G. Szokoly (Eoetvoes Univ., Budapest, and MPE) report for the GROND team:
We observed the field of GRB 081118 (Swift trigger 334877; Hoversten et al.,
GCN #8524) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla Observatory
(Chile). Observations started at 00:35 UT on November 19th, 2008, 9.6 hr
post-burst, when the GRB location was becoming visible at 20 degrees above
horizon. Observations were performed at an average seeing of 1.0".
We detect the source reported by D'Avanzo et al. (GCN #8528) at the edge of
the astrometrically corrected 2.0'' Swift-XRT error circle (Evans et al.,
GCN #8525). Based on a 20 min exposure at mid-time 1:55 UT, we estimate
preliminary magnitudes (all in AB system) of
g' ~ 23.3 mag,
r' ~ 23.0 mag,
i' ~ 22.5 mag,
z' ~ 22.0 mag,
J ~ 21.6 mag,
H ~ 21.2 mag and
K ~ 20.6 mag
with errors of +/- 0.2. These magnitudes are calibrated against GROND
zeropoints as well as 2MASS field stars, and are not corrected for the
galactic foreground extinction of E(B-V)= 0.04 (Schlegel et al. 1998).
The object clearly decayed by 0.6 mag over the course of 1.5 hrs, thus it
is suggested as the couterpart of GRB 081118.
The spectral energy distribution is consistent with a power law, so the
g'-band detection implies a redshift of this GRB smaller than 3.5.