Particle acceleration at magnetized, relativistic turbulent shock fronts

The efficiency of particle acceleration at shock waves in relativistic, magnetized astrophysical outflows is a debated topic with far-reaching implications. Here, for the first time, we study the impact of turbulence in the pre-shock plasma. Our simulations demonstrate that, for a mildly relativistic, magnetized pair shock (Lorentz factor $\gamma_{\rm sh} \simeq 2.7$, magnetization level $\sigma \simeq 0.01$), strong turbulence can revive particle acceleration in a superluminal configuration that otherwise prohibits it. Depending on the initial plasma temperature and magnetization, stochastic-shock-drift or diffusive-type acceleration governs particle energization, producing powerlaw spectra $\mathrm{d}N/\mathrm{d}\gamma \propto \gamma^{-s}$ with $s \sim 2.5-3.5$. At larger magnetization levels, stochastic acceleration within the pre-shock turbulence becomes competitive and can even take over shock acceleration

Deciphering the physical basis of the intermediate-scale instability

We study the underlying physics of cosmic-ray (CR) driven instabilities that play a crucial role for CR transport across a wide range of scales, from interstellar to galaxy cluster environments. By examining the linear dispersion relation of CR-driven instabilities in a magnetised electron-ion background plasma, we establish that both, the intermediate and gyroscale instabilities have a resonant origin and show that these resonances can be understood via a simple graphical interpretation. These instabilities destabilise wave modes parallel to the large-scale background magnetic field at significantly distinct scales and with very different phase speeds. Furthermore, we show that approximating the electron-ion background plasma with either magnetohydrodynamics (MHD) or Hall-MHD fails to capture the fastest growing instability in the linear regime, namely the intermediate-scale instability. This finding highlights the importance of accurately characterising the background plasma for resolving the most unstable wave modes. Finally, we discuss the implications of the different phase speeds of unstable modes on particle-wave scattering. Further work is needed to investigate the relative importance of these two instabilities in the non-linear, saturated regime and to develop a physical understanding of the effective CR transport coefficients in large-scale CR hydrodynamics theories.Comment: 14 pages, 3 figures, submitted to JPP Letters, comments welcom

Effects of reacceleration and source grammage on secondary cosmic rays spectra

The ratio between secondary and primary cosmic ray particles is the main source of information about cosmic ray propagation in the Galaxy. Primary cosmic rays are thought to be accelerated mainly in Supernova Remnant (SNR) shocks and then released in the interstellar medium (ISM). Here they produce secondary particles by occasional collisions with interstellar matter. As a result, the ratio between the fluxes of secondary and primary particles carries information about the amount of matter cosmic rays have encountered during their journey from their sources to Earth. Recent measurements by AMS-02 revealed an unexpected behaviour of two main secondary-to-primary ratios, the Boron-to-Carbon ratio and the anti-proton-to-proton ratio. In this work we discuss how such anomalies may reflect the action of two phenomena that are usually overlooked, namely the fact that some fraction of secondary particles can be produced within the acceleration region, and the non-negligible probability that secondary particles encounter an accelerator (and are reaccelerated) during propagation. Both effects must be taken into account in order to correctly extract information about CR transport from secondary-to-primary ratios

Accélération de particules dans les sources astrophysiques relativistes

Acceleration and dissipation in relativistic astrophysical sources It is generally accepted that the observed radiation from high-energy, powerful astrophysical sources fundamentally derives from the dissipation of the energy carried by an outflow into a population of accelerated particles. The work conducted in this PhD has focused on two generic mechanisms: particle acceleration at shock fronts and particle acceleration in turbulent plasmas, in the relativistic regime. To do so, we have conducted large-scale particle-in-cell (PIC) numerical simulations, which we have combined with analytical developments. In a first part, we have determined the saturation mechanism of the electromagnetic instability that governs the physics of weakly magnetised relativistic shocks. In a second part, we have tested against kinetic simulations a recent model of non-resonant particle acceleration in magnetized turbulence. Finally, in a last part, we have studied the interaction of a relativistic magnetised shock with a turbulent flow; we demonstrate, in particular, that this interaction can revive particle acceleration in a regime of magnetization in which shock acceleration was thought to be inefficient.Selon le paradigme usuel, le rayonnement non-thermique issu des sources astrophysiques de haute énergie résulte in fine de la dissipation d’un réservoir d’énergie en un gaz de particules accélérées. L’étude menée dans le cadre de ma thèse a porté sur deux mécanismes génériques : l’accélération de particules autour de fronts d’ondes de choc et l’accélération de particules dans les plasmas turbulents, dans le régime relativiste. À cette fin, nous avons conduit des simulations numériques particle-in-cell (PIC) à grande échelle, en parallèle de développements analytiques. Dans une première partie, nous avons déterminé le mécanisme de saturation de l’instabilité électromagnétique qui gouverne la physique des chocs relativistes, faiblement magnétisés. Dans une deuxième partie, nous avons testé à l’aide de simulations cinétiques un modèle récent d’accélération non-résonante dans une turbulence magnétisée. Enfin, dans une dernière partie, nous avons étudié l’interaction d’un choc magnétisé relativiste avec un plasma turbulent ; cela nous a notamment permis de montrer que cette interaction peut donner lieu à l’accélération de particules dans un régime de magnétisation dans lequel l’accélération autour d’ondes de choc semblait auparavant inefficace

Accélération de particules dans les sources astrophysiques relativistes

Acceleration and dissipation in relativistic astrophysical sources It is generally accepted that the observed radiation from high-energy, powerful astrophysical sources fundamentally derives from the dissipation of the energy carried by an outflow into a population of accelerated particles. The work conducted in this PhD has focused on two generic mechanisms: particle acceleration at shock fronts and particle acceleration in turbulent plasmas, in the relativistic regime. To do so, we have conducted large-scale particle-in-cell (PIC) numerical simulations, which we have combined with analytical developments. In a first part, we have determined the saturation mechanism of the electromagnetic instability that governs the physics of weakly magnetised relativistic shocks. In a second part, we have tested against kinetic simulations a recent model of non-resonant particle acceleration in magnetized turbulence. Finally, in a last part, we have studied the interaction of a relativistic magnetised shock with a turbulent flow; we demonstrate, in particular, that this interaction can revive particle acceleration in a regime of magnetization in which shock acceleration was thought to be inefficient.Selon le paradigme usuel, le rayonnement non-thermique issu des sources astrophysiques de haute énergie résulte in fine de la dissipation d’un réservoir d’énergie en un gaz de particules accélérées. L’étude menée dans le cadre de ma thèse a porté sur deux mécanismes génériques : l’accélération de particules autour de fronts d’ondes de choc et l’accélération de particules dans les plasmas turbulents, dans le régime relativiste. À cette fin, nous avons conduit des simulations numériques particle-in-cell (PIC) à grande échelle, en parallèle de développements analytiques. Dans une première partie, nous avons déterminé le mécanisme de saturation de l’instabilité électromagnétique qui gouverne la physique des chocs relativistes, faiblement magnétisés. Dans une deuxième partie, nous avons testé à l’aide de simulations cinétiques un modèle récent d’accélération non-résonante dans une turbulence magnétisée. Enfin, dans une dernière partie, nous avons étudié l’interaction d’un choc magnétisé relativiste avec un plasma turbulent ; cela nous a notamment permis de montrer que cette interaction peut donner lieu à l’accélération de particules dans un régime de magnétisation dans lequel l’accélération autour d’ondes de choc semblait auparavant inefficace

Particle acceleration in relativistic astrophysical sources

Selon le paradigme usuel, le rayonnement non-thermique issu des sources astrophysiques de haute énergie résulte in fine de la dissipation d’un réservoir d’énergie en un gaz de particules accélérées. L’étude menée dans le cadre de ma thèse a porté sur deux mécanismes génériques : l’accélération de particules autour de fronts d’ondes de choc et l’accélération de particules dans les plasmas turbulents, dans le régime relativiste. À cette fin, nous avons conduit des simulations numériques particle-in-cell (PIC) à grande échelle, en parallèle de développements analytiques. Dans une première partie, nous avons déterminé le mécanisme de saturation de l’instabilité électromagnétique qui gouverne la physique des chocs relativistes, faiblement magnétisés. Dans une deuxième partie, nous avons testé à l’aide de simulations cinétiques un modèle récent d’accélération non-résonante dans une turbulence magnétisée. Enfin, dans une dernière partie, nous avons étudié l’interaction d’un choc magnétisé relativiste avec un plasma turbulent ; cela nous a notamment permis de montrer que cette interaction peut donner lieu à l’accélération de particules dans un régime de magnétisation dans lequel l’accélération autour d’ondes de choc semblait auparavant inefficace.Acceleration and dissipation in relativistic astrophysical sources It is generally accepted that the observed radiation from high-energy, powerful astrophysical sources fundamentally derives from the dissipation of the energy carried by an outflow into a population of accelerated particles. The work conducted in this PhD has focused on two generic mechanisms: particle acceleration at shock fronts and particle acceleration in turbulent plasmas, in the relativistic regime. To do so, we have conducted large-scale particle-in-cell (PIC) numerical simulations, which we have combined with analytical developments. In a first part, we have determined the saturation mechanism of the electromagnetic instability that governs the physics of weakly magnetised relativistic shocks. In a second part, we have tested against kinetic simulations a recent model of non-resonant particle acceleration in magnetized turbulence. Finally, in a last part, we have studied the interaction of a relativistic magnetised shock with a turbulent flow; we demonstrate, in particular, that this interaction can revive particle acceleration in a regime of magnetization in which shock acceleration was thought to be inefficient

Saturation of the asymmetric current filamentation instability under conditions relevant to relativistic shock precursors

submitted to Physical Review EInternational audienceThe current filamentation instability, which generically arises in the counterstreaming of supersonic plasma flows, is known for its ability to convert the free energy associated with anisotropic momentum distributions into kinetic-scale magnetic fields. The saturation of this instability has been extensively studied in symmetric configurations where the interpenetrating plasmas share the same properties (velocity, density, temperature). In many physical settings, however, the most common configuration is that of asymmetric plasma flows. For instance, the precursor of relativistic collisionless shock waves involves a hot, dilute beam of accelerated particles reflected at the shock front and a cold, dense inflowing background plasma. To determine the appropriate criterion for saturation in this case, we have performed large-scale 2D particle-in-cell simulations of counterstreaming electron-positron pair and electron-ion plasmas. We show that, in interpenetrating pair plasmas, the relevant criterion is that of magnetic trapping as applied to the component (beam or plasma) that carries the larger inertia of the two; namely, the instability growth suddenly slows down once the quiver frequency of those particles equals or exceeds the instability growth rate. We present theoretical approximations for the saturation level. These findings remain valid for electron-ion plasmas provided that electrons and ions are close to equipartition in the plasma flow of larger inertia. Our results can be directly applied to the physics of relativistic, weakly magnetized shock waves, but they can also be generalized to other cases of study

Saturation of the asymmetric current filamentation instability under conditions relevant to relativistic shock precursors

submitted to Physical Review EThe current filamentation instability, which generically arises in the counterstreaming of supersonic plasma flows, is known for its ability to convert the free energy associated with anisotropic momentum distributions into kinetic-scale magnetic fields. The saturation of this instability has been extensively studied in symmetric configurations where the interpenetrating plasmas share the same properties (velocity, density, temperature). In many physical settings, however, the most common configuration is that of asymmetric plasma flows. For instance, the precursor of relativistic collisionless shock waves involves a hot, dilute beam of accelerated particles reflected at the shock front and a cold, dense inflowing background plasma. To determine the appropriate criterion for saturation in this case, we have performed large-scale 2D particle-in-cell simulations of counterstreaming electron-positron pair and electron-ion plasmas. We show that, in interpenetrating pair plasmas, the relevant criterion is that of magnetic trapping as applied to the component (beam or plasma) that carries the larger inertia of the two; namely, the instability growth suddenly slows down once the quiver frequency of those particles equals or exceeds the instability growth rate. We present theoretical approximations for the saturation level. These findings remain valid for electron-ion plasmas provided that electrons and ions are close to equipartition in the plasma flow of larger inertia. Our results can be directly applied to the physics of relativistic, weakly magnetized shock waves, but they can also be generalized to other cases of study

Particle acceleration at magnetized, relativistic turbulent shock fronts

International audienceThe efficiency of particle acceleration at shock waves in relativistic, magnetized astrophysical outflows is a debated topic with far-reaching implications. Here, for the first time, we study the impact of turbulence in the pre-shock plasma. Our simulations demonstrate that, for a mildly relativistic, magnetized pair shock (Lorentz factor $\gamma_{\rm sh} \simeq 2.7$, magnetization level $\sigma \simeq 0.01$), strong turbulence can revive particle acceleration in a superluminal configuration that otherwise prohibits it. Depending on the initial plasma temperature and magnetization, stochastic-shock-drift or diffusive-type acceleration governs particle energization, producing powerlaw spectra $\mathrm{d}N/\mathrm{d}\gamma \propto \gamma^{-s}$ with $s \sim 2.5-3.5$. At larger magnetization levels, stochastic acceleration within the pre-shock turbulence becomes competitive and can even take over shock acceleration

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