TITLE: GCN CIRCULAR NUMBER: 14117 SUBJECT: GRB 121229A: GROND detection of the afterglow DATE: 12/12/29 06:59:20 GMT FROM: Vladimir Sudilovsky at MPE K. Varela (MPE Garching), V. Sudilovsky (MPE Garching), S. Klose (TLS Tautenburg), and J. Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of GRB 121229A (Swift trigger 544347; Sonbas et al., GCN 14115) 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 29, 2012, at 05:09 UT, 540 s after the GRB trigger. They were performed at an average seeing of 1.0" and at an average airmass of 1.8. We find a point source at the border of the XRT error circle at a position RA (J2000.0) = 12:40:24.29 DEC (J2000.0) = -50:35:39.48 with an uncertainty of 0.5" in each coordinate. Based on a total exposure time of 1500 s in g'r'i'z' and 1200 s in JHK, at a midtime of 3840s after the burst, we estimate preliminary AB magnitudes of g' = 22.2 +/- 0.2, r' = 21.5 +/- 0.1, i' = 21.3 +/- 0.1, z' = 21.1 +/- 0.1, J = 20.9 +/- 0.1, H = 20.2 +/- 0.1, and K > 19.7. This source's spectral energy distribution is best fit with a power-law of slope beta = 0.8 +/- 0.2. We suggest that this source is the afterglow of GRB 121229A. Given magnitudes are calibrated against GROND zeropoints in g'r'i'z' and 2MASS field stars in JHK, and are not corrected for the Galactic foreground extinction corresponding to a reddening of E(B-V)= 0.16 mag in the direction of the burst (Schlegel et al. 1998).