Variation of bulk Lorentz factor in AGN jets due to Compton rocket in a complex photon field

TL;DR

This paper models how the complex photon environment around AGN influences jet acceleration via the Compton rocket effect, revealing variable bulk Lorentz factors and potential emission variability over different timescales.

Contribution

It introduces a detailed numerical analysis of the bulk Lorentz factor evolution in AGN jets considering multiple external photon sources and their geometries.

Findings

Bulk Lorentz factor exhibits acceleration and deceleration zones.

Jet patterns depend on source sizes and observation angles.

Variability in AGN emission can occur from hours to months.

Abstract

Radio-loud active galactic nuclei are among the most powerful objects in the universe. In these objects, most of the emission comes from relativistic jets getting their power from the accretion of matter onto supermassive black holes. However, despite the number of studies, a jet's acceleration to relativistic speeds is still poorly understood. It is widely known that jets contain relativistic particles that emit radiation through several physical processes, one of them being the inverse Compton scattering of photons coming from external sources. In the case of a plasma composed of electrons and positrons continuously heated by the turbulence, inverse Compton scattering can lead to relativistic bulk motions through the Compton rocket effect. We investigate this process and compute the resulting bulk Lorentz factor in the complex photon field of an AGN composed of several external photon sources. We consider various sources here: the accretion disk, the dusty torus, and the broad line region. We take their geometry and anisotropy carefully into account in order to numerically compute the bulk Lorentz factor of the jet at every altitude. The study, made for a broad range of parameters, shows interesting and unexpected behaviors of the bulk Lorentz factor, exhibiting acceleration and deceleration zones in the jet. We investigate the patterns of the bulk Lorentz factor along the jet depending on the source sizes and on the observation angle and we finally show that these patterns can induce variability in the AGN emission with timescales going from hours to months.