GCN Circular 20113
T. Kruehler and J. Greiner (both MPE Garching) report on behalf of the GROND team: We observed the field of GRB 161023A (INTEGRAL trigger 7613; Mereghetti et al., GCN #20103) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG telescope at the ESO La Silla Observatory (Chile). Observations started at 23:52 UT on 2016-10-24, 25.2 hr after the GRB trigger. They were performed at an average seeing of 1".6, and at an average airmass of 1.1. Based on combined images with 9.7 min of total exposures in g'r'i'z' and 10.0 min in JHK at a mid-time of 00:12 UT on 2016-10-25, we derive the following preliminary magnitudes (all in AB system) for the optical/NIR afterglow (Gorbovskoy et al. GCN 20102, Tanvir et al., GCN 20104, Guidorzi et al., GCN 20107). g' = 19.7 +- 0.1 mag r' = 19.4 +- 0.1 mag i' = 19.2 +- 0.1 mag z' = 19.1 +- 0.1 mag J = 18.7 +- 0.1 mag H = 18.4 +- 0.1 mag K = 18.2 +- 0.1 mag Given magnitudes are calibrated against GROND zeropoints as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.03 mag in the direction of the burst (Schlafly & Finkbeiner 2011).