TITLE: GCN CIRCULAR NUMBER: 18195 SUBJECT: GRB 150821A: GROND afterglow observations DATE: 15/08/22 04:11:28 GMT FROM: Thomas Kruehler at MPE Garching T. Kruehler (ESO), F. Schrey, W. Bornemann, A. Rau and J. Greiner (all MPE) report on behalf of a larger collaboration: We observed the field of GRB 150821A (Swift trigger 652847; Troja et al., GCN 18186) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile). Observations started at 02:02 UT on 2015-08-22, 16.3 hours after the GRB trigger. For the optical afterglow (Troja et al., GCN 18186, D'Elia et al. GCN 18187, Kuin & Troja, GCN 18192) we derive magnitudes of: g = 23.2 +- 0.1 r = 22.6 +- 0.1 i = 22.3 +- 0.1 z = 22.0 +- 0.2 at a midtime of 17.0 hours after the trigger. This photometry is in the AB system and calibrated against GROND zeropoints. Because GROND is presently being recommissioned, our measurements are preliminary.