Opacity effect on core-shift and the spectral properties of jets

Abstract

Context. There is theoretical and observational evidence that the jet core position changes with frequency. However, the core position for a given frequency may vary with time in the case of flares or for a precessing jet. Aims. In this work, we want to explore the changes in core position as a function of frequency, magnetic field alignment, relativistic electron density, and jet inclination angle. Methods. We use a physical model of a synchrotron-emitting jet. Two cases of the jet are analysed, namely with magnetic field parallel and perpendicular to the jet axis. The evolution of the related spectrum is monitored over the radio band. Results. We find that a smaller jet inclination angle or a higher electron density causes the jet core position to move downstream of the jet and we demonstrate that this displacement of the core along the jet gives rise to a spectral flattening.