Radiation from the Relativistic Jet: a Role of the Shear Boundary Layer

Abstract

Recent radio and optical large scale jets' observations suggest a two-component jet morphology, consisting of a fast central spine surrounded with a boundary layer with a velocity shear. We study radiation of electrons accelerated at such boundary layers as an option for standard approaches involving internal shocks in jets. The acceleration process in the boundary layer yields in a natural way a two component electron distribution: a power-law continuum with a bump at the energy, where energy gains equal radiation losses, followed by a cut-off. For such distributions we derive the observed spectra of synchrotron and inverse-Compton radiation, including comptonization of synchrotron and CMB photons. Under simple assumptions of energy equipartition between the relativistic particles and the magnetic field, the relativistic jet velocity at large scales and a turbulent character of the shear layer, the considered radiation can substantially contribute to the jet radiative output. In the considered conditions the synchrotron emission is characterized by a spectral index of the radio-to-optical continuum being approximately constant along the jet. A characteristic feature of the obtained broad-band synchrotron spectrum is an excess at X-ray frequencies, similar to the one observed in some objects by Chandra. As compared to the uniform jet models, the velocity shear across the radiating boundary region leads to decrease and frequency dependence of the observed jet-counterjet radio brightness asymmetry. We conclude that a careful investigation of the observational data looking for the derived effects can allow to evaluate the role of the boundary layer acceleration processes and/or impose constraints for the physical parameters of such layers in large scale jets.