TITLE: GCN CIRCULAR NUMBER: 13881 SUBJECT: GRB 121017A: GROND Observations DATE: 12/10/18 02:37:07 GMT FROM: Alexander Kann at TLS Tautenburg D. A. Kann (MPE/Universe Cluster), M. Nardini (Milano-Bicocca), and J. Greiner (MPE) report on behalf of the GROND team: We observed the field of GRB 121017A (Swift trigger 536172, Grupe et al., GCN # 13875) 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 October 17, 2012, at 23:50 UT, 4.45 hr after the GRB trigger. They were performed at an average seeing of 1" (in r') and at an average airmass of 1.3. We do not detect any sources in the enhanced XRT error circle (Goad et al., GCN # 13879), in agreement with earlier reports of optical observations (Jelínek et al., GCN # 13876, Volnova et al., GCN # 13877, Gorosabel & Mottola, GCN # 13878). There is a star at the eastern edge of the error circle but this affects only a small part of it. Based on 284 s of total exposures in g'r'i'z' (mean time 0.18978 days after the trigger) and 480 s in JHK (mean time 0.19003 days after the trigger) we estimate preliminary AB magnitudes (upper limits are 3 sigma) of g' > 23.2, r' > 23.2, i' > 22.4, z' > 22.2, J > 19.8, H > 19.4 and K > 19.3. Given magnitudes are calibrated against GROND zeropoints in the optical as well as 2MASS field stars in the NIR and are not corrected for the high Galactic foreground reddening of E_(B-V) = 0.44 mag (Schlegel et al. 1998).