Do the mildly superluminal VLBI knots exclude ultrarelativistic blazar jets?

Abstract

We compute the effective values of apparent transverse velocity and flux boosting factors for the VLBI radio knots of blazar jets by integrating over the angular distributions of these quantities across the widths of jets with finite opening angles but constant velocities. For high bulk Lorentz factors (Γ > 10), variations across the jet can be quite large if the opening angle, ω, is even a few degrees on subparsec scales. The resulting apparent speeds are often much lower than those obtained from the usual analyses that ignore the finite jet opening angles. We can thus reconcile the usually observed subluminal or mildly superluminal speeds with the very high (>20) Γ factors, required by the inverse Compton origin and rapid variability of TeV fluxes, as well as by intraday radio variability. Thus it is possible to associate the VLBI radio knots directly with shocks in the ultrarelativistic main jet flow, without invoking very rapid jet deceleration on parsec scales or extremely unlikely viewing angles.