Probing Acceleration and Turbulence at Relativistic Shocks in Blazar Jets

TL;DR

This paper uses multi-wavelength observations and detailed simulations to probe the turbulence and acceleration mechanisms in relativistic shocks within blazar jets, providing new constraints on particle diffusion and turbulence levels.

Contribution

It introduces a method to diagnose MHD turbulence and particle acceleration in blazar jets using spectral data and Monte Carlo simulations, offering novel insights into shock physics.

Findings

Synchrotron peak energy does not appear above 100 MeV in gamma-ray band.

Constraints on electron mean free paths and turbulence levels in blazar jets.

X-ray excess in AO 0235+164 explained by bulk Compton scattering, constraining diffusion coefficients.

Abstract

Diffusive shock acceleration (DSA) at relativistic shocks is widely thought to be an important acceleration mechanism in various astrophysical jet sources, including radio-loud active galactic nuclei such as blazars. Such acceleration can produce the non-thermal particles that emit the broadband continuum radiation that is detected from extragalactic jets. An important recent development for blazar science is the ability of Fermi-LAT spectroscopy to pin down the shape of the distribution of the underlying non-thermal particle population. This paper highlights how multi-wavelength spectra spanning optical to X-ray to gamma-ray bands can be used to probe diffusive acceleration in relativistic, oblique, magnetohydrodynamic (MHD) shocks in blazar jets. Diagnostics on the MHD turbulence near such shocks are obtained using thermal and non-thermal particle distributions resulting from detailed Monte Carlo simulations of DSA. These probes are afforded by the characteristic property that the synchrotron $\nu F_{\nu}$ peak energy does not appear in the gamma-ray band above 100 MeV. We investigate self-consistently the radiative synchrotron and inverse Compton signatures of the simulated particle distributions. Important constraints on the diffusive mean free paths of electrons, and the level of electromagnetic field turbulence are identified for three different case study blazars, Mrk 501, BL Lacertae and AO 0235+164. The X-ray excess of AO 0235+164 in a flare state can be modelled as the signature of bulk Compton scattering of external radiation fields, thereby tightly constraining the energy-dependence of the diffusion coefficient for electrons. The concomitant interpretations that turbulence levels decline with remoteness from jet shocks, and the probable significant role for non-gyroresonant diffusion, are posited.