GCN Circular 11478

F. Olivares E. (MPE Garching), A. Rossi (TLS Tautenburg), and J.
Greiner (MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 101219B (Swift trigger 440635; Gelbord et
al., GCN #11473) 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 during twilight at 00:33 UT on December 20, 8.1
hours after the GRB trigger, and were performed at an average seeing
of 1" and an average airmass of 1.05.

We confirm the optical afterglow candidate reported by Gelbord et
al.(GCN #11473). The afterglow faded, however, it is well detected in
all seven GROND optical/NIR bands.

Based on 141 s of total exposure in g'r'i'z' and 240 s in JHK, taken
at 00:36 UT mid-time, we estimate preliminary magnitudes (all in the
AB system) of:

g' = 20.4 +/- 0.1
r' = 20.0 +/- 0.1
i' = 20.0 +/- 0.2
z' = 19.6 +/- 0.1
J =�19.2 +/- 0.1
H =�18.8 +/- 0.1 and
K = 18.6 +/- 0.1

A second epoch was obtained at 03:10 UT mid-time, with total exposure
time of 1500 s in in g'r'i'z' and 1200 s seconds in JHK. The afterglow
has faded by ~0.3 mag in the r' and J bands, which yields a temporal
decay index of alpha = 1.0 between our two epochs.

Given magnitudes are calibrated against GROND zero points and 2MASS
field stars. The error in the optical bands is dominated
by the zero-point uncertainty. Magnitudes are not corrected for the
expected Galactic foreground extinction corresponding to a reddening
of E(B-V)=0.02 mag in the direction of the burst (Schlegel et al.
1998).