GCN Circular 20680

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
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 ) of GRB 170209A (Fermi/GBM trigger 508295323;
Roberts & Meegan 2017, GCN #20652
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 ) 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
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  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.