GCN Circular 15595

K. Varela, D.A. Kann and J. Greiner (all MPE Garching)
report on behalf of the GROND team:
  
We observed again the field of the GRB 131205A (Ukwatta et al., GCN #15578)
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 the La Silla Observatory (Chile).
  
Observations started at 05:22 UT on 11 December 2013. They were performed
at an average seeing of 0.7 arcsec and at an average airmass of 1.3.
  
Based on four co-added observations with individual exposure times of 2214s
in g'r'i'z' and 1800s in JHKs, we measure for the source at the border
of the X-ray error circle (Beardmore et al., GCN #15580), which was
proposed as potential counterpart (Schmidl et al. 2013, GCN #15585),
the following magnitudes:
  
r' = 26.5 +/- 0.4
i' = 25.3 +/- 0.4
  
Compared to our earlier measurements of r' = 24.8+/-0.2, i' = 24.7+/-0.3
this implies a fading of 1.7+/-0.5 in r', and 0.6+/-0.5 in i'. 
We conclude that the fading in r' is significant, and propose this
source as the optical counterpart of GRB 131205A.  The low amplitude
in the i'-band may be caused by the emergence of light from a red
host galaxy. Observations with bigger glasses are required to verify
this interpretation.
  
Note that the above r'i' magnitudes are calibrated against GROND zeropoints 
and are not corrected for the Galactic foreground extinction corresponding
to a reddening of E_(B-V)=0.25 mag in the direction of the burst (Schlegel
et al. 1998).