Abstract
Context.High-energy emission of extragalactic objects is known to take place in relativistic jets, but the nature, the location, and the emission processes of the emitting particles are still unknown. One of the models proposed to explain the formation of relativistic ejections and their associated non-thermal emission is the two-flow model, where the jets are supposed to be composed of two different flows, a mildly relativistic baryonic jet surrounding a fast, relativistically moving electron positron plasma. Here we present the simulation of the emission of such a structure taking into account the main sources of photons that are present in active galactic nuclei (AGNs).Aims.We try to reproduce the broadband spectra of radio-loud AGNs with a detailed model of emission taking into account synchrotron and inverse-Compton emission by a relativistically moving beam of electron positron, heated by a surrounding turbulent baryonic jet.Methods.We compute the density and energy distribution of a relativistic pair plasma all along a jet, taking into account the synchrotron and inverse-Compton process on the various photon sources present in the core of the AGN, as well as the pair creation and annihilation processes. We use semi-analytical approximations to quickly compute the inverse-Compton process on a thermal photon distribution with any anisotropic angular distribution. The anisotropy of the photon field is also responsible for the bulk acceleration of the pair plasma through the “Compton rocket” effect, thus imposing the plasma velocity along the jet. As an example, the simulated emerging spectrum is compared to the broadband emission of 3C 273.Results.In the case of 3C 273, we obtain an excellent fit of the average broadband energy distribution by assuming physical parameters compatible with known estimates. The asymptotic bulk Lorentz factor is lower than what is observed by superluminal motion, but the discrepancy could be solved by assuming different acceleration profiles along the jet.
Highlights
Radio-loud active galactic nuclei (AGNs) are known to be powerful emitters of non-thermal radiation
The goal of this paper is to present the complete calculation of the pair-beam non-thermal spectra assuming the above configuration, that is, (1) the pair plasma is assumed to be described by a pile-up distribution and is generated in situ by γ−γ interactions, (2) its motion is controlled at short and intermediate distances by the anisotropic Compton rocket effect in the complex photon field generated by an accretion disk, a broad line region (BLR), and a dusty torus, and (3) particles emit non-thermal radiation through synchrotron, synchrotron self-Compton (SSC), and external Compton (EC) in the various photon fields
Emission modelling is essential to understanding the AGN jet physics, and yet despite many flaws the one-zone model is still predominant
Summary
Radio-loud active galactic nuclei (AGNs) are known to be powerful emitters of non-thermal radiation. One-zone models are the most simple and widespread models for reproducing the AGN jet emission They assume a spherical, homogeneous emission zone with a minimal number of free parameters: the radius of the zone, the magnetic field, the bulk Lorentz factor, and parameters describing the particle distribution. They present the advantage of being simple and give a good first approximation of the physical conditions in jets. They have difficulties in reproducing the low-energy (radio) part of the spectral energy distribution (SED; most probably emitted by the farthest part of the jet) They assume the very stringent condition that the whole non-thermal emission must be produced in a single zone, which can be an issue for explaining the multi-wavelength variability of the sources. The strong Doppler boosting associated with highly relativistic jets is incompatible with the detection of high-energy emission from unbeamed radio galaxies seen at large angles, since their emission should be strongly attenuated by a Doppler factor smaller than one
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