D. A. Kann, M. Tanga (both MPE Garching), S. Klose, S. Schmidl (both TLS
Tautenburg) and J. Greiner (MPE Garching) report on behalf of the GROND
We observed the field of GRB 131218A (INTEGRAL-6409, Gotz et al., GCN
#15601) simultaneously in g'r'i'z'JHKs with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m MPG telescope at La Silla Observatory
A first series of observations (integration time 6750 sec in the optical,
5400 sec in the NIR, midtime 0.24 days after the trigger, seeing 1".0 and
airmass 1.5) yield the following image depths (AB magnitudes):
g' > 24.6,
r' > 24.9,
i' > 24.5,
z' > 24.4,
J > 22.4,
H > 21.8, and
K > 20.8.
A second set of observations (integration time 3750 sec in the optical,
3000 sec in the NIR, midtime 2.31 days after the trigger, seeing 0".9 and
airmass 1.3) yield somewhat deeper limits in the optical, while they are
somewhat shallower in the NIR.
Image subtraction between these two epochs shows no evidence for variable
sources, both in the INTEGRAL error circle and around it, and therefore we
can only place upper limits on the presence of any afterglow.
Magnitudes are calibrated against GROND zeropoints (g'r'i'z') and 2MASS
stars (JHK), and are not corrected for the Galactic foreground extinction
corresponding to a reddening of E_(B-V)=0.17 mag in the direction of the
burst (Schlegel et al. 1998).