TITLE: GCN CIRCULAR NUMBER: 9353 SUBJECT: GRB090510: VLT/FORS2 spectroscopic redshift DATE: 09/05/12 17:42:34 GMT FROM: Arne Rau at MPE Arne Rau (MPE Garching), Sheila McBreen (UCD/MPE Garching), Thomas Kruehler, and Jochen Greiner (both MPE Garching) report: We present follow-up optical spectroscopy of the short Swift/Fermi GRB 090510 (Hoversten et al., GCN 9331; Ohno & Pelassa, GCN 9334; Guiriec et al., GCN 9336) using VLT/FORS2 (PI McBreen). Observations of the optical source consistent with position of the afterglow (Marshall & Hoversten, GCN 9332; Olofsson et al., GCN 9338; Olivares et al., GCN 9352) started on May 12 08:17UT. A total of three 30min exposures with the 300I grism were obtained, covering an approximate wavelength range of 6000-10000 Angstroms. Preliminary analysis of the spectra reveals two emission lines at 7093 and 9250 Angstroms, which we identify as [OII] and Hbeta at a common redshift of z=0.903 +/- 0.003. Furthermore, there are indications of a spectral break in the continuum around 7600 Angstroms, suggesting a possible 4000A break at the same redshift. The detection of emission lines and the possible 4000A break suggest the dominance of the underlying host galaxy over the afterglow at the time of our observations. It also confirms that host galaxies of short GRBs are not all old and dead ellipticals but instead drawn from the underlying field galaxy distribution (e.g., Berger 2009, ApJ, 690, 231). In the acquisition image the center of the host is 0.7" offset with respect to the NOT refined afterglow position, which corresponds to 5.5 kpc at a z=0.903. At a redshift of z=0.903, the isotropic equivalent energy of the burst is 3.8E52 erg in the 1 keV - 10 MeV rest frame (using the Fermi/GBM spectral parameters of Guiriec et al., GCN 9336). The peak energy of the best fit GBM Band function model is 8.4 +/- 0.8 MeV in the rest frame. We acknowledge excellent support from the ESO staff, in particular F. Selman and I. Condor.