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

Recent radio and optical large scale jets' observations suggest atwo-component jet morphology, consisting of a fast central spine surroundedwith a boundary layer with a velocity shear. We study radiation of electronsaccelerated at such boundary layers as an option for standard approachesinvolving internal shocks in jets. The acceleration process in the boundarylayer yields in a natural way a two component electron distribution: apower-law continuum with a bump at the energy, where energy gains equalradiation losses, followed by a cut-off. For such distributions we derive theobserved spectra of synchrotron and inverse-Compton radiation, includingcomptonization of synchrotron and CMB photons. Under simple assumptions ofenergy equipartition between the relativistic particles and the magnetic field,the relativistic jet velocity at large scales and a turbulent character of theshear layer, the considered radiation can substantially contribute to the jetradiative output. In the considered conditions the synchrotron emission ischaracterized by a spectral index of the radio-to-optical continuum beingapproximately constant along the jet. A characteristic feature of the obtainedbroad-band synchrotron spectrum is an excess at X-ray frequencies, similar tothe one observed in some objects by Chandra. As compared to the uniform jetmodels, the velocity shear across the radiating boundary region leads todecrease and frequency dependence of the observed jet-counterjet radiobrightness asymmetry. We conclude that a careful investigation of theobservational data looking for the derived effects can allow to evaluate therole of the boundary layer acceleration processes and/or impose constraints forthe physical parameters of such layers in large scale jets.Comment: 30 pages, 4 figures included. Modified version, accepted for publication in Astrophysical Journa