TITLE: GCN CIRCULAR NUMBER: 14091 SUBJECT: GRB 121217A: GROND Detection of the Afterglow DATE: 12/12/17 13:32:56 GMT FROM: Sebastian Schmidl at TLS Tautenburg J. Elliott (MPE Garching), S. Schmidl (TLS Tautenburg), and J.Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of GRB 121217A (Swift trigger 542441; Siegel et al., GCN 14089) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La Silla Observatory (Chile). Observations started on December 17, 2012, at 07:21 UT, 210 s after the GRB trigger. They were performed at an average seeing of 1.6" and at an average airmass of 1.2. We found a fading point source at the border of the enhanced XRT error circle given in Evans et al. (GCN 14090) RA (J2000.0) = 10:14:50.407 DEC (J2000.0) = -62:21:03.53 with an uncertainty of 0.5" in each coordinate. Based on a total exposure time of 456 s in g'r'i'z' and 480 s in JHK, at a midtime of 1882 s after the burst, we estimate preliminary AB magnitudes of g' = 20.3 +/- 0.1, r' = 19.3 +/- 0.1, i' = 18.9 +/- 0.1, z' = 18.6 +/- 0.1, J = 18.1 +/- 0.1, H = 17.7 +/- 0.1, K = 17.4 +/- 0.1. In the r' band the source faded with a decay slope of 0.7 +/- 0.1 between 2000 s and 4200 s after the trigger. The SED can be fit with a power-law with a slope of beta = 0.8 +/- 0.1. Given magnitudes are calibrated against GROND zeropoints as well as 2MASS field stars and are not corrected for the Galactic foreground extinction corresponding to a reddening of E(B-V)= 0.38 mag in the direction of the burst (Schlegel et al. 1998). The extinction-corrected SED suggests a photo-z of 3.0 +/- 0.2. We caution, however, that the high Galactic extinction along the line of sight might affect this result.