γ-rays in flat-spectrum AGN: revisiting the fast jet hypothesis with the CJF sample

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: mkarouzos@mpifr-bonn.mpg.de
² I.Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 23 October 2010
Accepted: 2 February 2011

Abstract

Context. The recent release of the First Fermi-LAT Source Catalog solidified the predominant association of extragalactic γ-ray emitters to active galaxies, in particular blazars. A tight connection between AGN jet kinematics and γ-ray properties has been argued for, attributing the energetic emission from active galaxies to their highly relativistic outflows.

Aims. We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample to study the connection between AGN jet kinematics and their γ-ray properties. The high number of sources included in the sample, in addition to the excellent kinematic data available, allows us to investigate the origin of γ-ray emission in AGN.

Methods. We identify the CJF sources detected in γ-rays (by Fermi-LAT and EGRET). We use γ-ray luminosities and the available VLBI kinematic data to look for correlations between γ-ray and kinematic properties, as well as for differences between AGN classes (quasars, BL Lacs, radio galaxies). We also check the kinematics of the TeV sources in the CJF.

Results. 21.8% of the CJF has been detected in the γ-ray regime. We find the detectability of BL Lacs significantly higher compared to quasars. Sources detected in the γ-rays show a wider apparent jet velocity distribution compared to the non-detected ones, but the maxima of both distributions are at similar values. No strong link between γ-ray detectability and fast apparent jet speeds is found. A tentative correlation is found between γ-ray luminosity and maximum apparent jet speeds, stronger for BL Lac and γ-variable sources. We find non-radial jet motions to be important to γ-ray emission. We suggest two-zone, spine-sheath models as a possible explanation for our results. Two out of four CJF TeV sources show superluminal jet speeds, in contrast to previous studies.