Synchrotron emission from relativistic parsec – scale jets

We developed theory of particle acceleration inside the relativistic rotating electronpositron force–free jet with spiral magnetic field. We considered perturbation of stationary magnetic field structure and found that acceleration takes place in the regions where the Alfvén resonant condition with the eigenmodes in the jet is fulfilled, i.e. where the local Alfvén speed is equal to the phase speed of an eigenmode. Acceleration process and synchrotron losses combined together form power law energy spectrum of ultrarelativistic electrons and positrons with index between 2 and 3 depending upon initial energy of injected particles. Synchrotron emission of these electrons and positrons in spiral magnetic field of rotating force–free jet has been calculated. Polarization properties of the radiation has been obtained and compared with existing VLBI polarization measurements of parsec–scale jets in BL Lac sources and quasars. Our results give natural explanation of observed bimodality in alignment between electric field vector and jet axis. Degree of polarization and velocity of observed proper motion of bright knots depend upon angular rotational velocity of the jet. Thus, comparing them to each other, we can estimate angular rotational velocity in jets. These results indicate that the fact, that generally in BL Lac objects electric field vector is oriented parallel to the jet axis while in quasars perpendicular to the jet axis, may be due to intrinsically larger angular rotational velocity and large winding of magnetic field in BL Lac jets than in quasar jets.