TITLE:   GCN CIRCULAR
NUMBER:  22840
SUBJECT: GRB 180624A: GROND observations
DATE:    18/06/25 08:09:51 GMT
FROM:    Patricia Schady at MPE/Swift  <pschady@mpe.mpg.de>

Patricia Schady (MPE Garching) reports:

We observed the field of GRB 180624A (Swift trigger 844192; Gibson et al.,
GCN #22832) 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 04:42 UT on 2018-06-25, 14.9 hrs after the GRB
trigger. They were performed under poor seeing conditions and at an
average airmass of 1.5.

A source is visible within the 1.5" enhanced Swift-XRT error circle
reported by GOAD et al. (GCN #22835), and consistent with the position of
the optical afterglow candidate reported by Guidorzi et al. (GCN #22839).
The source appears extended and most of the flux in the JHK bands is to
the NW of the  afterglow error circle.

Based on a total of 22 min exposure time in g'r'i'z', we estimate
preliminary AB magnitudes of

g' = 21.5 +/- 0.1 mag
r' = 20.2 +/- 0.1 mag
i' = 19.9 +/- 0.1 mag
z' = 19.6 +/- 0.1 mag

A fit to the GROND optical SED implies a photometric redshift of
z_phot=3.75+/-0.10.

Given magnitudes are calibrated against SDSS field stars and are not
corrected for the expected Galactic foreground extinction corresponding to
a reddening of E_(B-V)=0.04 in the direction of the burst (Schlafly &
Finkbeiner 2011).

We thank Markus Rabus for the excellent support from La Silla.