On the redshift of the bright BL Lacertae object PKS 0048-097

Aims: We attempt to determine the spectroscopic redshifts of bright BL Lac objects, which have proven difficult to measure.
Methods: 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.
Results: Our observations of PKS 0048-097 detect three emission lines, which permit us to derive a redshift z = 0.635. Moreover, we discover a Mg II absorption system at z = 0.154 that is associated with a foreground spiral galaxy a 50 kpc of projected distance.
Conclusions: The measured redshift allows us to discuss the optical beaming factor and the absorption of the high energy spectrum by the extragalactic background light.