Relativistic Collisionless Shocks in Inhomogeneous Magnetized Plasmas

Relativistic collisionless shocks are associated with efficient particle acceleration when propagating into weakly magnetized homogeneous media; as the magnetization increases, particle acceleration becomes suppressed. We demonstrate that this changes when the upstream carries kinetic-scale inhomogeneities, as is often the case in astrophysical environments. We use fully-kinetic simulations to study relativistic perpendicular shocks in magnetized pair plasmas interacting with upstream density perturbations. The upstream fluctuations are found to corrugate the shock front and generate large-scale turbulent shear motions in the downstream, which in turn are capable of accelerating particles. This can revive relativistic magnetized shocks as viable energization sites in astrophysical systems, such as jets and accretion disks. The generation of large-scale magnetic structures also has important implications for polarization signals from blazars.Comment: 8 pages, 5 figure

Shock corrugation to the rescue of the internal shock model in microquasars: The single-scale MHD view

Questions regarding energy dissipation in astrophysical jets are open to date, despite of numerous attempts to limit the diversity of models. Some of the most popular models assume that energy is transferred to particles via internal shocks, which develop as a consequence of non-uniform velocity of the jet matter. In this context, we study the structure and energy deposition of colliding plasma shells, focusing our attention on the case of initially inhomogeneous shells. This leads to formation of distorted (corrugated) shock fronts -- a setup that has recently been shown to revive particle acceleration in relativistic magnetized perpendicular shocks. Our studies show that the radiative power of the far downstream of non-relativistic magnetized perpendicular shocks is moderately enhanced with respect to the flat shock cases. Based on the decay rate of downstream magnetic field, we make predictions for multiwavelength polarization properties.Comment: 17 pages, 8 figures; Accepted for publication in Ap

Chocs, turbulence et accélération de particules dans le contexte de la magnétohydrodynamique relativiste : études numériques et théoriques

Quels sont les processus physiques à l’origine des propriétés non-thermiques observées dans les sources astrophysiques de haute énergie ? D’où viennent les rayons cosmiques de haute énergie ? Les réponses à ces questions semblent être intimement liées à la physique des chocs, de la turbulence et de l’accélération de particules. Notamment, comprendre les chocs non-collisionnels, qui sont parmi les sites d’accélération les plus activement explorés par l’astrophysique moderne, implique de rendre compte de dépendances complexes entre ces trois composantes.Nous plaçant ici dans le cadre de travail de la magnétohydrodynamique (MHD) relativiste, nous examinons tour à tour différents aspects impliqués dans la physique non-linéaire et multi-échelle qui régit ces systèmes. Dans un premier temps, nous traitons d’un problème en lien avec la problématique des interactions choc-turbulence, à savoir la réponse d’un choc rapide perpendiculaire à des ondes magnétohydrodynamiques en provenance de l’amont du choc. Nous démontrons numériquement que cette réponse est dans certaines conditions résonantes, comme cela avait été prédit par une étude linéaire dans la limite relativiste. Par le biais de simulations bidimensionnelles à haute résolution effectuées avec un code à maillage adaptatif, MPI-AMRVAC, nous sondons ce phénomène dans les régimes sub-relativiste et relativiste ainsi que son évolution non-linéaire. On se concentre ensuite sur le problème de particules test interagissant avec une turbulence MHD relativiste pour étudier de manière analytique et numérique la physique de l’accélération stochastique, en insistant sur les spécificités du régime relativiste, qui demeure peu exploré. Au niveau analytique, nous dérivons des expressions pour les coefficients de diffusion de l’angle d’attaque et de la quantité de mouvement pour différents modèles phénoménologiques de turbulence largement acceptés dans la littérature. Nous nous affranchissons de certaines limites de la théorie quasi-linéaire standard en incorporant des effets d’élargissement de résonance dus à la décorrélation des ondes qui composent la turbulence et à des perturbations non-linéaires de la trajectoire des particules sujettes aux réflexions de miroirs magnétiques. Nous montrons que ces estimations analytiques sont en bon accord avec nos simulations de particules test évoluant dans une turbulence prescrite. Enfin, on présente les premiers résultats de nos simulations de MHD relativiste tridimensionnelles de turbulence forcée avec évolution temporelle, utilisées pour explorer l’accélération stochastique dans un plasma relativiste chaud de magnétisation de l’ordre de l’unité. On trouve en particulier que le coefficient de diffusion en quantité de mouvement a une dépendance Dpp ~p2, ce qui est cohérent avec nos prédictions analytiques pour une turbulence faite de perturbations de type ondes d’Alfvén et avec de récentes simulations « Particule en cellule » ayant sondé un régime similaire.What are the physical processes underlying the non-thermal features observed in high-energy astrophysical sources? What are the origins of high-energy cosmic rays? The answers to these questions seem to be intimately related to the physics of shocks, turbulence and particle acceleration. Notably, understanding collisionless shocks, which are among the most prominent potential acceleration sites considered in modern astrophysics, implies to account for a complex interplay between these three components. Relying on the framework of special relativistic magnetohydrodynamics (SRMHD), we investigate in turn different aspects involved in the non-linear and multi-scale physics governing such systems. First, we examine a problem related to the issue of shock-turbulence interactions; namely the response of a perpendicular fast shock to upstream magnetohydrodynamic waves and demonstrate numerically that this response can be resonant, as predicted by a recent linear study in the relativistic limit. By means of high resolution two-dimensional SRMHD simulations carried out with the adaptive mesh code MPI-AMRVAC, we probe this phenomenon in the relativistic and sub-relativistic regimes, as well as its non-linear evolution. We then shift the focus towards the problem of test particles interacting with SRMHD turbulence to investigate analytically and numerically the physics of stochastic acceleration, emphasizing the specifities of the relativistic regime, which remains largely unexplored. On the analytic level, we provide expressions for the quasi-linear pitch angle and momentum diffusion coefficients for widely accepted phenomenological models of MHD turbulence, going beyond standard quasi-linear theory by incorporating resonance broadening effects due to the decorrelation of the waves composing the turbulence and non-linear perturbations to the trajectories of particles subjected to magnetic mirroring. These analytical estimations are shown to be in good agreement the results of our simulations of test particles involving in synthetic turbulence. Finally, we introduce the first results from three-dimensional time evolving SRMHD simulations of driven turbulence, used to probe stochastic acceleration in relativistically hot plasmas with magnetization of order unity. We derive in particular momentum diffusion coefficients scaling as Dpp ~p2 consistent with our analytic predictions for turbulence made of Alfven like perturbations and recent Particle-In-Cell simulations, which explored a similar regime

Particle acceleration in relativistic turbulence: a theoretical appraisal

Phys. Rev. D, submitted; 22 pages, 11 figures; comments welcomeWe discuss the physics of stochastic particle acceleration in relativistic MHD turbulence, combining numerical simulations of test-particle acceleration in synthetic wave turbulence spectra with detailed analytical estimates. In particular, we study particle acceleration in wave-like isotropic fast mode turbulence, in Alfv\'en and slow Goldreich-Sridhar type wave turbulence (properly accounting for local anisotropy effects), including resonance broadening due to wave decay and pitch-angle randomization. At high particle rigidities, the contributions of those three modes to acceleration are comparable to within an order of magnitude, as a combination of several effects (partial disappearance of transit-time damping for fast modes, increased scattering rate for Alfv\'en and slow modes due to resonance broadening). Additionally, we provide analytical arguments regarding acceleration beyond the regime of MHD wave turbulence, addressing the issue of non-resonant acceleration in a turbulence comprised of structures rather than waves, as well as the issue of acceleration in small-scale parallel electric fields. Finally, we compare our results to the existing literature and provide ready-to-use formulas for applications to high-energy astrophysical phenomenology

Relativistic magnetohydrodynamical simulations of the resonant corrugation of a fast shock front

The generation of turbulence at magnetized shocks and its subsequent interaction with the latter is a key question of plasma- and high-energy astrophysics. This paper presents two-dimensional magnetohydrodynamic simulations of a fast shock front interacting with incoming upstream perturbations, described as harmonic entropy or fast magnetosonic waves, both in the relativistic and the sub-relativistic regimes. We discuss how the disturbances are transmitted into downstream turbulence and we compare the observed response for small amplitude waves to a recent linear calculation. In particular, we demonstrate the existence of a resonant response of the corrugation amplitude when the group velocity of the outgoing downstream fast mode matches the velocity of the shock front. We also present simulations of large amplitude waves to probe the non-linear regime

Nonresonant particle acceleration in strong turbulence: Comparison to kinetic and MHD simulations

International audienceCollisionless, magnetized turbulence offers a promising framework for the generation of nonthermal high-energy particles in various astrophysical sites. Yet, the detailed mechanism that governs particle acceleration has remained subject to debate. By means of 2D and 3D particle-in-cell, as well as 3D (incompressible) magnetohydrodynamic (MHD) simulations, we test here a recent model of nonresonant particle acceleration in strongly magnetized turbulence [Lemoine, Phys. Rev. D 104, 063020 (2021)], which ascribes the energization of particles to their continuous interaction with the random velocity flow of the turbulence, in the spirit of the original Fermi model. To do so, we compare, for a large number of particles that were tracked in the simulations, the predicted and the observed histories of particles momenta. The predicted history is that derived from the model, after extracting from the simulations, at each point along the particle trajectory, the three force terms that control acceleration: the acceleration of the field line velocity projected along the field line direction, its shear projected along the same direction, and its transverse compressive part. Overall, we find a clear correlation between the model predictions and the numerical experiments, indicating that this nonresonant model can successfully account for the bulk of particle energization through Fermi-type processes in strongly magnetized turbulence. We also observe that the parallel shear contribution tends to dominate the physics of energization in the particle-in-cell simulations, while in the magnetohydrodynamic incompressible simulation, both the parallel shear and the transverse compressive term provide about equal contributions