T. Kruehler (DARK), J. Greiner (MPE) and D. A. Kann (TLS) report:
We performed deep observations of the field of GRB 100728A
simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405). GRB 100728A triggered Swift (Cannizzo et al.,
GCN 11004), GBM (von Kienlin, GCN 11006), Konus-Wind (Golenetskii
et al., GCN 11021) and Suzaku WAM (Tsai et al., GCN 11037).
Our late observations were performed on 2010-11-10, 105 days
after the trigger, and consisted of dithered images with a total of
4427 s exposure in g'r'i'z' and 3600 s in JHK under clear sky
conditions and a seeing of 0.8".
Inside the enhanced X-ray error-circle
(http://www.swift.ac.uk/xrt_positions), we detect an extended
object with an r'-band AB magnitude of r' = 25.0 +/- 0.2.
Its barycentric coordinates are:
RA (J2000) = 05:55:01.98
Dec. (J2000) = -15:15:19.7
with uncertainties of 0.3" in each coordinate.
Registering the early afterglow images (Olivares et al., GCN 11020)
against the late host frame, we derive an offset between galaxy
center and afterglow of approximately 0.4".
A spectrum of this galaxy was taken on 2012-11-15 with the ESO VLT
UT2 equipped with the X-shooter spectrograph, covering the
wavelength range 3000-25000 AA. The seeing was 1.2" and the
total exposure of our spectrum was 2400 s. In the VIS and NIR arm,
we detect emission lines, which we interpret as being due to
[O II](3729) and Halpha at a common redshift of z = 1.567.
The bright X-ray emission (Evans et al., GCN 11014), coupled with the
faint and red optical/NIR afterglow (Olivares et al., GCN 11020) is
characteristic of dust-extinguished events. Fitting the available
afterglow data with synchrotron models, the visual extinction along
the GRB sightline is constrained to 1.5 mag < A_V < 3 mag at z = 1.567.