Exploring the bulk of the BL Lac object population: parsec scale radio properties and gamma ray emission

G Giovannini,And A Mignano,G Tosti,M Kadler,B Boccardi,C Casadio,S Tamburri,M Giroletti,E Liuzzo,G.B Taylor

doi:10.1051/epjconf/20136108006

Abstract

BL Lac objects have been found to be the largest population of emitters in gamma-ray band. However, since they are relatively weak radio sources, their parsec scale structure and most of their radio properties are poorly known. To increase our knowledge of the BL Lac object population, we selected a sample of BL Lacs from the BZ Cat at low redshift (z < 0.2), with no constrain on the radio flux density and gamma-ray activity. We present here the results of a first VLBA observation at 8 and 15 GHz and shortly discuss their properties

Highlights

Based on results from the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory (CGRO), it is well known that the main contribution to the γ-ray sky comes from the Galactic plane emission, pulsars, and blazars [11]
This difference cannot be due to the radio spectrum or to variability, and it suggests the presence of a sub-kiloparsec radio structure lost in VLBA data because of sensitivity and the lack of short baselines
To better understand this point, we evaluated, for all objects, the source compactness (SC) value defined as the ratio of the 8 GHz VLBA and the NVSS total powers

Summary

Introduction

Based on results from the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory (CGRO), it is well known that the main contribution to the γ-ray sky comes from the Galactic plane emission, pulsars, and blazars [11]. Among the latter class of objects, Flat Spectrum Radio Quasars (FSRQs) were the most numerous compared to the BL Lac objects, making up 77% of the high confidence blazar associations. Thanks to Fermi findings, the high energy characteristics of BL Lacs can be investigated, but to understand the origin and the nuclear properties of these sources more information at different frequencies are necessary.

Objectives

Results

Conclusion