TITLE: GCN CIRCULAR NUMBER: 14500 SUBJECT: GRB 100728A: GROND host detection and X-shooter redshift DATE: 13/04/29 11:32:33 GMT FROM: Thomas Kruehler at Dark Cosmology Center 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.