GCN Circular 14049
T. Kruehler (DARK/NBI), A. Nicuesa Guelbenzu (TLS Tautenburg) and J. Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of the Swift GRB 121209A (Maselli et al., GCN 14045) simultaneously in g'r'i'z'JHKs with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG/ESO telescope at La Silla Observatory (Chile). Observations started on December 10, 2012, 00:24 UT which is roughly 2.4 hr after the trigger. Within the UVOT-enhanced XRT position (see link in Maselli et al., GCN 14045), we detect a faint source in the stack of all g'r'i'z' data at: RA (J2000) = 21:47:08.96 Dec (J2000) = -08:14:06.3 with an estimated uncertainty of 0.5" in each coordinate. Forcing photometry on this position, we estimate preliminary AB magnitudes and upper limits in the individual filters of: g' > 24.0 mag r' = 23.9 +- 0.3 mag i' = 23.6 +- 0.4 mag z' = 23.2 +- 0.3 mag J > 21.5 mag H > 21.0 mag Ks > 19.5 mag These magnitudes and upper limits are based on images with a mid-time of 00:59 UT and an effective integration time of 26.4 min in griz and 24.0 min in the JHKs filters. They are calibrated against SDSS and 2MASS field stars and have not been corrected for the expected Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.05 mag (Schlegel et al. 1998). No statement about variability can be made at this point.