Radiation drag in relativistic active galactic nucleus jets

We present the first comprehensive study of radiation drag imposed on both cold and relativistic plasma components in a jet propagating at relativistic speeds through a dense radiation field. Earlier work showed that radiation drag acting on the cold plasma alone drops very fast with distance from the central source and can affect the jet dynamics only very close to the central source (up to 10–30 gravitational radii), and only if the jet plasma is strongly dominated by electron-positron pairs. In the case of radiation drag acting on a jet through relativistic electrons/positrons, the distance dependence can be much flatter, actually being determined by the distance dependence of the injection rate of relativistic particles. However, we show that such radiation drag can be dynamically important only for magnetically dominated jets in which the injection luminosity of relativistic electrons/positrons exceeds the kinetic luminosity of the jet. Our studies are performed using a radiation tensor formalism, which is the most suitable and elegant method for calculating both forces and cooling rates resulting from Compton scattering of an ambient radiation field by relativistic plasma in a jet. Simple analytic formulae, derived using this method, allow one to make quick estimates of dynamical and radiative effects resulting from the interaction of relativistic jets with different external radiation field distributions.