Microphysics of Particle Reflection in Weibel-Mediated Shocks

Particle-in-cell (PIC) simulations have shown that relativistic collisionless shocks mediated by the Weibel instability accelerate about 1% of incoming particles, while the majority are transmitted through the shock and become thermalized. The microphysical processes that determine whether an incoming particle will be transmitted or reflected are poorly understood. We study the microphysics of particle reflection in Weibel-mediated shocks by tracking a shell of test particles in a PIC simulation of a shock in pair plasma. We find that electrons in positron-dominated filaments and positrons in electron-dominated filaments efficiently reflect off of strong magnetic structures at the shock. These reflected particles headed towards the upstream must then find filaments of the same sign of current as the current carried by the reflected particles in order to successfully move with the shock and participate in diffusive shock acceleration (DSA). The final injection efficiency on the order of about 1% thus results from the effectiveness of the initial reflection at the shock and the reflected particles' probability of survival in the upstream post-reflection. We develop a model that predicts the fraction of high-energy particles as a function of the properties of Weibel filamentation.Comment: 14 pages, 9 figures, submitted to ApJ. Comments welcom

Origin of intense electron heating in relativistic blast waves

The modeling of gamma-ray burst afterglow emission bears witness to strong electron heating in the precursor of Weibel-mediated, relativistic collisionless shock waves propagating in unmagnetized electron-ion plasmas. In this Letter, we propose a theoretical model, which describes electron heating via a Joule-like process caused by pitch-angle scattering in the decelerating, self-induced microturbulence and the coherent charge-separation field induced by the difference in inertia between electrons and ions. The emergence of this electric field across the precursor of electron-ion shocks is confirmed by large-scale particle-in-cell (PIC) simulations. Integrating the model using a Monte Carlo-Poisson method, we compare the main observables to the PIC simulations to conclude that the above mechanism can indeed account for the bulk of electron heating.Comment: 9 pages, 8 figures; to be published in Astrophysical Journal Letter

The proton inelastic cross section at ultrahigh energies

We study the consequences of high-energy collider data on the best fits to total, elastic, and inelastic cross sections for $pp$ and $p\bar{p}$ scattering using two very distinct unitarisation schemes: the eikonal and the $U$-matrix. Despite their analytic differences, we find that the two schemes lead to almost identical predictions up to EeV energies, with differences only becoming significant at GUT-scale and higher energies.Comment: 7 pages, 4 figure

Unitarisation dependence of diffractive scattering in light of high-energy collider data

peer reviewedWe study the consequences of high-energy collider data on the best fits to total, elastic, inelastic, and single-diffractive cross sections for pp and pp¯ scattering using different unitarisation schemes. We find that the data are well fitted both by eikonal and U-matrix schemes, but that diffractive data prefer the U-matrix. Both schemes may be generalised by means of an additional parameter; however, this yields only marginal improvements to the fits. We provide estimates for ρ, the ratio of the real part to the imaginary part of the elastic amplitude, for the different fits. We comment on the effect of the different schemes on present and future cosmic ray data

Études théoriques et numériques des ondes de choc relativistes et sans collisions

Collisionless relativistic shock waves play a major role in extreme astrophysical objects such as gamma-ray bursts, blazars and pulsars wind nebulae, in which they are held responsible for producing nonthermal particle and radiation distributions. Without an external magnetic field, these shocks stem from the interaction, mediated by microinstabilities, of a beam of Fermi-accelerated particles with the ambient plasma. There results an electromagnetic turbulence that scatters both the beam and plasma particles. While the background plasma is mainly slowed down and heated, a fraction of its particles are accelerated to suprathermal energies, thus sustaining the shock wave. Understanding the highly nonlinear physics of such structures requires combining analytical models and large-scale particle-in-cell (PIC) numerical simulations. After a short review of the concepts and numerical techniques used to address the topic, we first examine the evolution of the current filamentation instability, which prevails in the precursor region of initially unmagnetized shocks. We then develop a comprehensive microphysical model of such shocks. To this purpose, we introduce the notion of a preferential frame, in which the microturbulence is quasi-magnetostatic, thus allowing the description of the particle scattering to be greatly simplified. Finally, we analyze the influence of a neutron ejecta propagating upstream of a gamma-ray burst shock. For each study, our model predictions are substantiated by state-of-the-art PIC simulations. particulesLes ondes de choc relativistes et sans collisions jouent un rôle majeur dans la physique des objets astrophysiques extrêmes, tels que les sursauts gamma, les blazars ou les nébuleuses de vent de pulsar, au sein desquels elles contribuent à la production de distributions non thermiques de particules et rayonnement. Ces ondes de choc résultent de l'interaction, par l'entremise d'une turbulence électromagnétique engendrée par des micro-instabilités, entre un faisceau de particules accélérées par processus de Fermi et le plasma ambiant. La modélisation de leur dynamique constitue un problème complexe, dont le traitement requiert de combiner analyse théorique et simulations numériques de type particle-in-cell (PIC). Après un résumé des concepts et des outils numériques nécessaires à la modélisation du problème, nous étudions l'évolution non-linéaire de l'instabilité de filamentation de courant qui domine la physique du précurseur de tels chocs. Dans un second temps, nous développons un modèle complet de la micro-physique de ces chocs, basé sur la définition d'un référentiel privilégié dans lequel la turbulence est quasi magnétostatique. Ce référentiel nous permet de caractériser le chauffage et le ralentissement du plasma de fond ainsi que la dynamique du faisceau. Pour terminer, nous explorons l'effet d'un éjecta neutronique sur l'évolution du choc avant dans un sursaut gamma. Pour chaque étude, nos prédictions théoriques sont étayées par des simulations PIC de haute résolution

Fast radio bursts as precursor radio emission from monster shocks

International audienceIt has been proposed recently that the breaking of MHD waves in the inner magnetosphere of strongly magnetized neutron stars can power different types of high-energy transients. Motivated by these considerations, we study the steepening and dissipation of a strongly magnetized fast magnetosonic wave propagating in a declining background magnetic field, by means of particle-in-cell simulations that encompass MHD scales. Our analysis confirms the formation of a monster shock as $B^2-E^2 \to 0$, that dissipates about half of the fast magnetosonic wave energy. It also reveals, for the first time, the generation of a high-frequency precursor wave by a synchrotron maser instability at the monster shock front, carrying a fraction of $\sim 10^{-3}$ of the total energy dissipated at the shock. The spectrum of the precursor wave exhibits several sharp harmonic peaks, with frequencies in the GHz band under conditions anticipated in magnetars. Such signals may appear as fast radio bursts

Fast radio bursts as precursor radio emission from monster shocks

International audienceIt has been proposed recently that the breaking of MHD waves in the inner magnetosphere of strongly magnetized neutron stars can power different types of high-energy transients. Motivated by these considerations, we study the steepening and dissipation of a strongly magnetized fast magnetosonic wave propagating in a declining background magnetic field, by means of particle-in-cell simulations that encompass MHD scales. Our analysis confirms the formation of a monster shock as $B^2-E^2 \to 0$, that dissipates about half of the fast magnetosonic wave energy. It also reveals, for the first time, the generation of a high-frequency precursor wave by a synchrotron maser instability at the monster shock front, carrying a fraction of $\sim 10^{-3}$ of the total energy dissipated at the shock. The spectrum of the precursor wave exhibits several sharp harmonic peaks, with frequencies in the GHz band under conditions anticipated in magnetars. Such signals may appear as fast radio bursts

Microphysics of Particle Reflection in Weibel-mediated Shocks

Particle-in-cell (PIC) simulations have shown that relativistic collisionless shocks mediated by the Weibel instability accelerate ∼1% of incoming particles, while the majority are transmitted through the shock and become thermalized. The microphysical processes that determine whether an incoming particle will be transmitted or reflected are poorly understood. We study the microphysics of particle reflection in Weibel-mediated shocks by tracking a shell of test particles in a PIC simulation of a shock in pair plasma. We find that electrons in positron-dominated filaments and positrons in electron-dominated filaments efficiently reflect off of strong magnetic structures at the shock. To participate in diffusive shock acceleration, however, these reflected particles headed toward the upstream must avoid getting advected downstream. This is enabled by incoming filaments, which trap reflected particles carrying the same sign of current as the filaments. The final injection efficiency on the order of ∼1% thus results from the effectiveness of the initial reflection at the shock and the reflected particles’ probability of survival in the upstream postreflection. We develop a model that predicts the fraction of high-energy particles as a function of the properties of Weibel filamentation

Origin of intense electron heating in relativistic blast waves

International audienceThe modeling of gamma-ray burst afterglow emission bears witness to strong electron heating in the precursor of Weibel-mediated, relativistic collisionless shock waves propagating in unmagnetized electron–ion plasmas. In this Letter, we propose a theoretical model, which describes electron heating via a Joule-like process caused by pitch-angle scattering in the decelerating, self-induced microturbulence and the coherent charge-separation field induced by the difference in inertia between electrons and ions. The emergence of this electric field across the precursor of electron–ion shocks is confirmed by large-scale particle-in-cell (PIC) simulations. Integrating the model using a Monte Carlo-Poisson method, we compare the main observables to the PIC simulations to conclude that the above mechanism can indeed account for the bulk of electron heating

Stability analysis of a periodic system of relativistic current filaments

International audienceThe nonlinear evolution of current filaments generated by the Weibel-type filamentation instability is a topic of prime interest in space and laboratory plasma physics. In this paper, we investigate the stability of a stationary periodic chain of nonlinear current filaments in counterstreaming pair plasmas. We make use of a relativistic four-fluid model and apply the Floquet theory to compute the two-dimensional unstable eigenmodes of the spatially periodic system. We examine three different cases, characterized by various levels of nonlinearity and asymmetry between the plasma streams: a weakly nonlinear symmetric system, prone to purely transverse merging modes; a strongly nonlinear symmetric system, dominated by coherent drift-kink modes whose transverse periodicity is equal to, or an integer fraction of the unperturbed filaments; a moderately nonlinear asymmetric system, subject to a mix of kink and bunching-type perturbations. The growth rates and profiles of the numerically computed eigenmodes agree with particle-in-cell simulation results. In addition, we derive an analytic criterion for the transition between dominant filament-merging and drift-kink instabilities in symmetric two-beam systems