Relativistic Effects in Extragalactic Radio Sources

This thesis deals with the interpretation of compact extragalactic radio sources in the context of the twin-beam model with a particular emphasis on the role of special-relativistic kinematical and dynamical effects. Various such effects are investigated, both in a general manner and in application to specific observed phenomena. Among the observed phenomena which are interpreted in this way are: i) Apparent superluminal flux variations, i.e., variations which occur on timescales short compared with the light travel time across the apparent source. It is shown that this and other spectral effects could be attributed to relativistic expansion of the source, and that objections raised against the relativistic interpretation (Terrell 1977) are model-dependent and do not apply in general. ii) Apparent superluminal separation of source components. It is shown that in a relativistic jet model where the radio emission originates both from the quasi-steady jet itself and from behind shock waves which travel with relativistic speeds in the jet, the source could display apparent superluminal expansion in which the moving component (associated with a shock) and the stationary component (associated with the optically-thick core of the jet) would have comparable, Doppler-boosted fluxes. The shocks could either propagate in the jet or arise behind dense clouds which are accelerated by the supersonic flow. The origin as well as the kinematical and radiative properties of the clouds are discussed, and it is proposed that the bright knots observed in certain jets, such as the jet in M87, could in fact correspond to dense accelerated clouds. iii) Rapid swings in the observed polarization position angle. It is shown that synchrotron sources which accelerate to (or decelerate from) relativistic speeds could display rapid swings in the observed polarization position angle of up to 180° as a result of the relativistic aberration effect. This mechanism is suggested for the large swing observed in the BL Lac object AO 0235+164 (Ledden and Aller 1978). It is argued that the majority of bright compact sources are observed along lines of sight making small (≲ 10°) angles to the jet velocity. On the basis of this hypothesis, a unified interpretation of compact, variable radio sources and of extended double radio sources is presented. More generally, it is suggest­ed that active galactic nuclei may have two types of emission: an isotropic, fairly steady, unpolarized optical continuum, and a beamed, variable, strongly polarized synchrotron component associated with a relativistic jet. The sequence: radio-quiet quasars, radio-loud quasars, and blazars (i.e., optically-violent variable quasars and BL Lac objects) would then correspond to similar sources which are viewed at progressively decreasing angles to their jet axes. Model synchrotron and inverse-Compton spectra for resolved jets and for unresolved inhomogeneous jets are calculated, taking into account the effect of synchrotron - radiation losses, and applied to the interpretation of the spectra of BL Lac objects. The possible contribution of beamed sources to the diffuse x-ray and γ-ray background is also discussed. In addition, the steady two-dimensional flow of an ideal compressible fluid is studied in the context of special-relativ­istic gas dynamics. The Newtonian equations for potential flow are generalized, and it is found that they can have the same form in the Newtonian and the relativistic regimes if their parameters are defined in the local rest-frame of the fluid. The Mach number thus defined is shown to have the same properties as its Newtonian analog. The Newtonian equations for oblique plane shock waves are similarly generalized in certain cases (which include, in particular, the· extreme-relativistic limit). Several applications of these results, particularly to the study of relativistic jets, are suggested.