R. Yates (MPE Garching) , D. A. Kann (TLS Tautenburg), C. Delvaux, and J.
Greiner (both MPE Garching) report on behalf of the GROND team:
We observed the field of GRB 151205B (Swift trigger 666378; Marshall et
al., GCN #18659) simultaneously in g'r'i'z'JHK with GROND (Greiner et al.
2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla
Observations started at 01:21 UT on 06/12/2015, 3hrs 38 mins after the GRB
trigger, and continued for several hours. We obtained a second epoch in
the following night.
We stacked the four deepest images of the first night (exposure times 7920
s in g'r'i'z' and 6600 s in JHK). They were performed at an average seeing
of 0".77 and an average airmass of 1.04. The observations of the second
night (exposure times 5760 s in g'r'i'z' and 5400 s in JHK) were obtained
at 0".62 seeing and airmass 1.04.
As there is no XRT detection for this GRB (Kennea et al., GCN #18661;
D'Avanzo et al., GCN #18672), we performed image subtraction. We note that
the g'r'i'z' images do not cover about 25% of the revised BAT error circle
(Ukwatta et al., GCN #18670) to the north and east. The JHK images cover
both the flight and the revised BAT error circle completely.
No significantly variable sources are detected in any band. The formal
upper limits (all in AB system) of our images, centered at 0.1939 days
after the GRB, are:
g' > 26.5 mag,
r' > 26.5 mag,
i' > 25.6 mag,
z' > 25.0 mag,
J > 22.7 mag,
H > 22.2 mag, and
K > 21.3 mag.
The given limits are derived based on calibrating the images against GROND
zero points and 2MASS field stars and are not corrected for the Galactic
foreground extinction corresponding to a reddening of E_(B-V)= 0.02 in the
direction of the burst (Schlegel et al. 1998).