2022MNRAS.509.1899N

Geometry of relativistic jets in active galaxies provides important information about mechanisms of launching, collimation, and acceleration of plasma flow. We propose a new method to probe a boundary shape of a jet on parsec scales - in the vicinity of its radio core. Apparent speed of an outflow is derived from variability time delays and core shifts measured at the same jet region, providing a self-consistent estimate of the Lorentz factor Γ. We link together the distance along the jet z with its transverse size assuming a constant flow acceleration. Our results indicate that jets have parabolic shape and sustain an effective acceleration in the core region, consistent with the Lorentz factor dependency Γ ∝ z^0.5. The proposed method can be applied to the sources observed at small viewing angles as well as to the distant sources when direct measurements are impossible due to a limited angular resolution.