TITLE: GCN CIRCULAR NUMBER: 20680 SUBJECT: GRB 170209A: GROND confirmation of MASTER afterglow DATE: 17/02/15 16:29:28 GMT FROM: Jochen Greiner at MPI P. Schady, J. Greiner (both MPE Garching), and S. Steinmassl (TU Munich), report: We observed the field of the optical transient reported by MASTER (Podesta et al. 2017, GCN #20650) of GRB 170209A (Fermi/GBM trigger 508295323; Roberts & Meegan 2017, GCN #20652) 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 03:16 UT on 2017-02-10, 26 hrs after the GRB trigger, resulting in 36 min of total exposures in g'r'i'z' and 30 min in JHK. They were performed at an average seeing of 2.3" and at an airmass of 1.1. A second epoch observation was started at 03:03 UT on 2017-02-15, around 6 days after the GRB trigger, resulting in 72 min of total exposures in g'r'i'z' and 60 min in JHK. Seeing conditions were better, around 1.3". We detect two sources close to the MASTER position: the fainter one, to the North-West, has faded by more than 1.2 mag, while the second source (to the South-West) stayed constant within the errors. The position of the fading object is RA (2000.0) = 07:23:07.19 Decl.(2000.0) = -52:14:45.5 with an error of +-0.3", and about 1.5 arcsec from the MASTER position. We find the following magnitudes of the fading object: epoch 1 epoch 2 g' > 23.5 mag > 24.6 mag r' = 23.4 +- 0.1 mag > 24.6 mag i' = 23.3 +- 0.3 mag > 24.2 mag z' = 23.1 +- 0.3 mag > 24.2 mag J > 21.3 mag > 21.7 mag H > 21.0 mag > 21.3 mag K > 19.3 mag > 19.7 mag The close match with the MASTER object, the fading and the powerlaw-like r'i'z colors confirm the nature of this object as the afterglow of GRB 170209A, as suggested also by Hurley et al. 2017, GCN #20656 based purely on the positional coincidence. Given magnitudes are calibrated against GROND zeropoints and 2MASS field stars and are not corrected for the Galactic foreground extinction corresponding to a reddening of E(B-V)= 0.12 mag in the direction of the burst (Schlafly & Finkbeiner 2011). We thank the support astronomer, S. Ciceri, for the support of the observations.