Linear size–extended radio luminosity (D–P E ) correlation in BL Lacertae objects: evidence for large scale beaming?

Abstract

In this paper, we use the distributions of projected linear size (D), core- (P C ) and extended- (P E ) radio luminosities, to investigate a consequence of relativistic beaming and radio source orientation scenario for low-luminosity extragalactic radio sources. In this scenario, BL Lacertae objects (BL Lacs) are believed to be Fanaroff-Riley type I (FR I) radio galaxies, but with radio axes aligned close to the line of sight. At this orientation, the core emission is greatly enhanced by relativistic Doppler boosting and linear size foreshortened due to geometrical projection. A simple outcome of this scenario is that the extended luminosity is expected to be orientation invariant, but a D–P C correlation is envisaged. Results show that both the relative core dominance (R) and linear size are strongly correlated with extended luminosity (r≥ 0.7). Using the R-distribution and R–P E anti-correlation, we show that the difference in radio core-dominance between FR I radio galaxies and X-ray selected BL Lacs can be accounted for by a bulk Lorentz factor γ∼5–13 and viewing angle ϕ∼5–15∘, which can be understood in terms of the scenario, with relativistic beaming persisting at largest scales.