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

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: 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.

A inhomogenous jet model for the broad band emission of radio loud AGNs

“…In this model, the three main sources of external photons in AGN are present: the accretion disc, the dusty torus and the broad line region (BLR). Their geometry is the same used and described in [26].…”

“…As explained in the section 2, the dynamic of the jet is driven by the Compton rocket process [15] in the two-flow paradigm as described here. Therefore, the bulk Lorentz factor of the jet Γ b (z) is not a free parameter of the model but as shown by [26] is imposed by the external sources. We use the method described in [26] to compute the evolution of Γ b (z) along the jet thanks to the external sources geometry described previously and to the physical conditions in the jet.…”

“…Subsequent works [12,13,20,21,6,26] have developed the two-flow paradigm, enriched by theoretical developments and numerical developments.…”

On radiative acceleration in spine–sheath structured blazar jets

“…As the seed photons of Compton scattering are produced outside the layer, if the scattering particles are hot in K , the scattered radiation is anisotropic also in the layer co-moving frame, making the layer recoil. For this reason, this interaction is often called Compton Rocket (Sikora et al, 1996;Vuillaume, Henri & Petrucci, 2015) and for hotter particles this driving force increases proportionally to their average internal energy γ 2 .…”

Apparent superluminal velocities and random walk in the velocity space

“…Possible acceleration mechanisms are mostly related to the propagation of relativistic shock waves and turbulent structures in the jets and include diffusive shock acceleration (first-order Fermi acceleration), fast magnetic reconnection, second-order Fermi acceleration (stochastic acceleration), Compton rocket [9,[13][14][15]. There is, probably, a competition between different acceleration mechanisms, with stochastic acceleration playing a significant role [13].…”