TITLE: GCN CIRCULAR NUMBER: 13976 SUBJECT: GRB 121117A: GROND confirmation of the UVOT afterglow candidate DATE: 12/11/19 22:35:54 GMT FROM: Sylvio Klose at TLS Tautenburg A. Nicuesa Guelbenzu, S. Klose (both TLS Tautenburg), M. Nardini (Universita degli studi di Milano-Bicocca), and J.Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of GRB 121117A (Swift trigger 538696, Cummings et al., GCN #13967) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG/ESO telescope at La Silla Observatory (Chile). First-epoch observations started on November 18, 2012, at 00:16 UT, about 15.5 hrs after the GRB trigger. Second-epoch observations were performed the following night. On the first-epoch images we find a faint source within the r=1.7 arcsec XRT error circle (Beardmore et al., GCN 13972) at coordinates RA(J2000) = 02:06:26.58, Dec. = 07:25:10.2 (+/- 0".3). Within the astrometric error this is consistent with the object seen by UVOT in the white band during the first 200 seconds after the GRB trigger (Oates et al., GCN 13973). For this source we measure the following preliminary AB magnitudes: g' > 24.0, r' = 23.6 +/- 0.2, i' = 23.0 +/- 0.2, z' > 23.1, J > 21.3, H > 20.8, K > 19.5, calibrated against SDSS and 2MASS field stars. On the second-epoch images the object had faded to r'=24.5 +/- 0.2 and i'>24.3, implying that this was the GRB afterglow.