GCN Circular 21729

P. Schady (MPE Garching) reports

I observed the field of GRB 170827A (Swift trigger 769567; Marshall et
al., GCN #21718) 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 00:44 UT on 28-08-2017, 19.5 hours after the GRB
trigger. They were performed at an average seeing of 1.5" and at an
average airmass of 1.0.

A point source is present within the 4.9" Swift-XRT error circle (Marshall
et al., GCN #21718) in the g'r'i'z' GROND images, consistent with the
ePESSTO position reported by Lyman et al. (GCN #21728).

Based on a total exposure time of 36 min in g'r'i'z' and 30 min in JHK,
the estimated preliminary magnitudes and upper limits (all in AB system)
are

g' = 22.8 +/- 0.1 mag
r' = 21.9 +/- 0.1 mag
i' = 21.6 +/- 0.1 mag
z' = 21.4 +/- 0.1 mag
J > 21.1 mag
H > 20.6 mag
K > 20.1 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)=0.09 mag in the
direction of the burst (Schlafly & Finkbeiner 2011).

A second fainter source is also detected within the XRT error circle at
around 2.3" west of the ePESSTO afterglow candidate. No statement can be
made at this time on the variability of either source.

I acknowledge the excellent help provided by the support astronomer in La
Silla, Markus Rabus, in obtaining these data.