2012A&A...543A.116L

We attempt to determine the spectroscopic redshifts of bright BL Lac objects, which have proven difficult to measure. We use the spectra acquired using the newly available spectrograph X-Shooter at the European Southern Observatory Very Large Telescope, which combines high resolution with a large wavelength range, to obtain ultraviolet to near-infrared spectra of BL Lacs. Our observations of PKS 0048-097 detect three emission lines, which permit us to derive a redshift z=0.635. Moreover, we discover a MgII absorption system at z=0.154 that is associated with a foreground spiral galaxy a 50kpc of projected distance. The measured redshift allows us to discuss the optical beaming factor and the absorption of the high energy spectrum by the extragalactic background light.