TITLE:   GCN CIRCULAR
NUMBER:  21178
SUBJECT: GROND observations of  GRB 170531B
DATE:    17/06/01 09:42:36 GMT
FROM:    Patricia Schady at MPE/Swift  <pschady@mpe.mpg.de>

P. Schady and T. Kruehler (both MPE) report

We observed the field of GRB 170531B (Swift trigger 755354; Evans et al.,
GCN #21171) 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 03:55 UT on 01-06-2017, 5.9 hours the GRB trigger.
They were performed at an average seeing of 1.3" and at an average airmass
of 1.3.

We detect a point source within the 1.8" enhanced Swift/XRT error circle
(Osborne et al., GCN #21175), and consistent with the afterglow position
reported by Ugarte Postigo et al. (GCN #21177).

Based on a total exposure of 54 min in g'r'i'z' and 45 min in JHK, we
estimate preliminary magnitudes (all in AB) of

g' = 22.65 +/- 0.04
r' = 22.15 +/- 0.04
i' = 21.99 +/- 0.05
z' = 21.84 +/- 0.06
J = 21.8 +/- 0.3
H = 21.5 +/- 0.3
K > 19.9

Given magnitudes are calibrated against PanSTARRS and 2MASS field stars
and are not corrected for the expected Galactic foreground extinction
corresponding to a reddening of E_(B-V)=0.16 mag in the direction of the
burst (Schlafly & Finkbeiner 2011).

We acknowledge the excellent support provided by the Chilean observer at
the telescope, and the support astronomer, Sam Kim, in obtaining these
data.