Exploring the bulk of the BL Lac object population:1. parsec-scale radio structures

Abstract

Context. The advent of Fermi is changing our understanding on the radio and gamma-ray emission in Active Galactic Nuclei. Contrary to pre-Fermi ideas, BL Lac objects are found to be the most abundant emitters in the gamma-ray band. However, since they are relatively weak radio sources, most of their parsec-scale structure and their multi-frequency properties are poorly understood and/or have not been investigated in a systematically fashion. Aims. Our main goal is to analyze the radio and gamma-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 gamma-ray flux densities/emission. Methods. Thanks to new Very Long Baseline Array observations at 8 and 15 GHz for the whole sample, we present here fundamental parameters such as radio flux densities, spectral index information, and parsec-scale structure. Moreover, we search for gamma-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.

19 pages, 6 figures, 6 tables, accepted for publication in A&A

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Cited by in corpus (4)

• HESS J1943+213: a non-classical high-frequency-peaked BL Lac object
• The loud and the quiet: searching for radio counterparts of two radio-weak BL Lac candidates with VLBI
• e-MERLIN observations of the puzzling TeV source HESS J1943+213
• Exploring the bulk of the BL Lac object population. II. Gamma-ray properties