GCN Circular 19959

P Wiseman (MPE Garching), J Bolmer (ESO Vitacura), and J Greiner (MPE
Garching) report on behalf of the GROND team:

We observed the field of GRB 160927A (Swift trigger 713782; Gibson et al.,
GCN #19952) 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 23:26 UT on 27/09/2016, 5h 21m after the GRB
trigger, and continued for ~1 hour, mostly during astronomical twilight.
They were performed at an average seeing of 1.8" and at an average airmass
of 2.

Based on 22.4 min of total exposures around a mid-time of 5.58 hours after
the trigger we marginally detect the source reported by Tkachenko et al.
(GCN #19954) and D'Avanzo et al. (GCN#19956) in the GROND r�-band with an
AB magnitude of:

r' = 23.9 +/- 0.4 mag.

The apparent fading since the RTT150 and TNG observations is convincing
evidence for this source being the afterglow, and further suggests a
steepening of the afterglow light-curve since the TNG observation.

The given magnitude is calibrated against SDSS zeropoints and is not
corrected for the expected Galactic foreground extinction corresponding to
a reddening of E_(B-V)=0.07 mag in the direction of the burst (Schlafly &
Finkbeiner, 2011).