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

¹ Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
e-mail: marco.landoni@uninsubria.it
² INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy
³ INAF – Osservatorio Astronomico di Padova, Vicolo dell’ Osservatorio 5, 35122 Padova, Italy
⁴ INAF – Istituto Nazionale di Astrofisica and INFN, Istituto Nazionale di Fisica Nucleare, Italy
⁵ Max-Planck-Institut fur Astronomie, Konigstuhl 17, 69117 Heidelberg, Germany
⁶ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland

Received: 25 February 2012
Accepted: 26 April 2012

Abstract

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.