Numerical modeling and physical interplay of stochastic turbulent acceleration for non-thermal emission processes

TL;DR

This paper introduces a new Eulerian algorithm integrated into a relativistic MHD code to model the combined effects of shock and turbulent acceleration on high-energy particle emissions in astrophysical jets.

Contribution

A novel Eulerian framework for incorporating stochastic turbulent acceleration alongside diffusive shock acceleration in RMHD simulations.

Findings

Turbulent acceleration produces more diffuse high-energy emission.

Shock acceleration localizes emission near shock regions.

The combined model reveals the interplay between acceleration mechanisms in jets.

Abstract

Particle acceleration is an ubiquitous phenomenon in astrophysical and space plasma. Diffusive shock acceleration (DSA) and stochastic turbulent acceleration are known to be the possible mechanisms for producing very high energetic particles, particularly in weakly magnetized regions. An interplay of different acceleration processes along with various radiation losses is typically observed in astrophysical sources. While DSA is a systematic acceleration process that energizes particles in the vicinity of shocks, stochastic turbulent acceleration (STA) is a random energizing process, where the interaction between cosmic ray particles and electromagnetic fluctuations results in particle acceleration. This process is usually interpreted as a biased random walk in energy space, modelled through a Fokker-Planck equation. In the present work, we describe a novel Eulerian algorithm, adopted to incorporate turbulent acceleration in the presence of DSA and radiative processes like synchrotron and Inverse-Compton emission. The developed framework extends the hybrid Eulerian-Lagrangian module in a full-fledged relativistic Magneto-hydrodynamic (RMHD) code PLUTO. From our validation tests and case studies, we showcase the competing and complementary nature of both acceleration processes. Axisymmetric simulations of an RMHD jet with this extended hybrid framework clearly demonstrate that emission due to shocks is localized while that due to turbulent acceleration originates in the backflow and is more diffuse, particularly in the high energy X-ray band.