Exploring the bulk of the BL Lacertae object population

I. Parsec-scale radio structures^⋆

¹ INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
e-mail: liuzzo@ira.inaf.it
² Dipartimento di Fisica e Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Max Planck Institute for Radioastronomy, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ INAF-Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
⁵ Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
⁶ Department of Physics and Astronomy, University of New Mexico, Albuquerque NM 87131, USA
⁷ Instituto de Astrofisica de Andalucia, CSIC, Apartado 3004, 18080 Granada, Spain
⁸ Universität Würzburg¨, Lehrstuhl für Astronomie, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
⁹ Dipartimento di Fisica, Università degli Studi di Perugia, 06123 Perugia, Italy

Received: 25 June 2013
Accepted: 11 September 2013

Abstract

Context. The advent of Fermi is changing our understanding on the radio and γ-ray emission in active galactic nuclei. Unlike pre-Fermi ideas, BL Lac objects are found to be the most abundant emitters in the γ-ray band. However, since they are relatively weak radio sources, most of their parsec-scale structure and their multifrequency properties are poorly understood and/or have not been investigated in a systematic fashion.

Aims. Our main goal is to analyze the radio and γ-ray emission properties of a sample of 42 BL Lacs selected, for the first time in the literature, with no constraint on their radio and γ-ray emission.

Methods. Thanks to new Very Long Baseline Array observations at 8 and 15 GHz for the whole sample, we present fundamental parameters here such as radio flux densities, spectral index information, and parsec-scale structure. Moreover, we search for γ-ray counterparts using data reported in the Second Catalog of Fermi Gamma-ray sources.

Results. Parsec-scale radio emission is observed in the majority of the sources at both frequencies. Gamma-ray counterparts are found for 14/42 sources.

Conclusions. The comparison between our results in radio and gamma-ray bands points out the presence of a large number of faint BL Lacs showing “non-classical” properties such as low source compactness, core dominance, no gamma-ray emission, and steep radio spectral indexes. A deeper multiwavelength analysis will be needed.