Effects of Jet Opening Angle and Velocity Structure on Blazar Parameters

TL;DR

This study investigates how jet opening angles and velocity structures affect observed blazar parameters, revealing that wider jets and complex velocity profiles lead to lower apparent speeds and Doppler factors, explaining observational differences in TeV blazars.

Contribution

It provides a detailed analysis of how jet opening angles and velocity structures influence observed blazar properties, extending previous models to include flux-weighted viewing angles and different jet velocity profiles.

Findings

Jet opening angles significantly reduce apparent speeds and Doppler factors.

Highly relativistic jets with wider opening angles appear less superluminal.

TeV blazars' low superluminal speeds can be explained by their jets' relativistic speeds and weaker collimation.

Abstract

We had earlier shown that for a constant velocity jet the discrepancy between the low speeds indicated by VLBI knot motions and the high Doppler factors inferred from emission of TeV photons could be reconciled if ultrarelativistic jets possessed modest opening angles. Here we evaluate the (flux-weighted) viewing angles of the jet and the apparent speeds and Doppler factors of the radio knots on parsec scales. The influence of the jet opening angle on these radio knot parameters are found for the usually considered types of relativistic nuclear jets: those with uniform bulk speeds and those where the bulk Lorentz factor of the flow decreases with distance from the jet axis, known as `spine--sheath' flows. For both types of jet velocity structures the expectation value of the jet orientation angle at first falls dramatically with increases in the (central) jet Lorentz factor, but for extremely relativistic jets it levels off at a fraction of the opening angle. The effective values of the apparent speeds and Doppler factors of the knots always decline substantially with increasing jet opening angle. The rarity of highly superluminal parsec-scale radio components in TeV blazars can be understood if their jets are both highly relativistic and intrinsically weaker, so probably less well collimated, than the jets in ordinary blazars.