arXiv subjects

Martin Lemoine

Publications and source records attributed to Martin Lemoine.

At least 19 recordsLinked to original sources

arXiv2026

Self-Consistent Modelling of Neutrino Production in Turbulent Black Hole Coronae

Stochastic particle acceleration in magnetized turbulent plasmas has emerged as a key mechanism to explain multi-messenger signals from compact astrophysical environments. Self-consistent modelling remains challenging because it requires to treat simultaneously several non-linear kinetic processes, especially turbulence-driven acceleration and its feedback on the turbulent cascade, as well as the radiative and hadronic losses, including the reprocessing of electromagnetic radiation in radiatively dense environments. The present paper introduces the hybrid numerical code Turb-AM3 designed to this effect. This hybrid numerical code couples the state-of-the-art time-dependent lepto-hadronic radiative solver AM3 with a stochastic acceleration module that incorporates recent theoretical advances in turbulent acceleration and accounts for the dynamical damping of turbulence by accelerated particles. In the second part of the paper, we use this code to provide self-consistent time-dependent models of proton acceleration in the turbulent black hole corona of NGC~1068. We find that the IceCube neutrino signal is well reproduced for a standard set of physical parameters describing the black hole corona. The same template model accounts in a satisfactory way for IceCube observations of other active galactic nuclei. Furthermore, our exploration of parameter space allows us to predict detailed template spectral shapes for the TeV neutrino spectrum, which in turn help understand how future neutrino observations can constrain the properties of turbulent AGN coronae and the underlying acceleration mechanism. This Turb-AM3 framework provides a powerful tool to model multi-messenger emission in a broad variety of compact astrophysical environments.

astro-ph.HE↗

arXiv2025

On the origin of ~ 100 TeV neutrinos from the Seyfert galaxy NGC 7469

The origin of TeV-PeV neutrinos detected by IceCube remains largely unknown. The most significant individual neutrino source is the close-by Seyfert galaxy NGC 1068 at 4.2$\sigma$ level with a soft spectral index. Another notable candidate is the Seyfert galaxy NGC 7469, which has been recently proposed as a potential neutrino emitter. The likelihood fit of the IceCube data for this source returned a very hard spectral index of ~ 1.9 and the excess is dominated by two high-energy events, issued as two neutrino alerts IC220424A and IC230416A. The energies of the two neutrinos are estimated to be 100-200 TeV, implying a maximum proton energy > 2 PeV, significantly higher than that in NGC 1068. The lack of lower-energy neutrinos from NGC 7469 also suggests a neutrino spectrum harder than that of NGC 1068. In this paper, we analyze the Fermi-LAT observations of NGC 7469, which yield non-detection. By requiring the cascade flux accompanying neutrino production not to exceed the upper limit of the GeV flux, the size of the neutrino-emitting region can be constrained when the neutrino flux takes a high value of the allowed range. We suggest that protons are accelerated to PeV energies via turbulence or magnetic reconnection in the corona of NGC 7469 and interact with OUV photons from the accretion disk and X-rays from the corona through the $p\gamma$ process, producing neutrinos with energy of 100-200 TeV. In the turbulence acceleration scenario, the required maximum proton energy can be achieved with a magnetization parameter close to unity ($\sigma\sim 1$), while in the reconnection scenario, a magnetization parameter with $\sigma\sim 10$ is needed. In both scenarios, a pair dominated composition for the corona is preferred. The difference in the neutrino spectrum between NGC 7469 and NGC 1068 could be due to a different magnetization despite that they belong to the same type of AGN.

astro-ph.HE↗

arXiv2025

Interpreting the multi-TeV emission from GRB 221009A with a second electron component accelerated by turbulence in the jet

The detection of very high-energy (VHE) afterglow emission of the gamma-ray burst (GRB) 221009A by the Large High Altitude Air Shower Observatory (LHAASO) provides a unique opportunity to probe particle acceleration in relativistic outflows. The hard spectrum at multi-TeV band cannot be fully explained by synchrotron-self-Compton radiation of the conventional one-zone afterglow model. In this work, we introduce a second component of relativistic electrons from stochastic acceleration via downstream turbulence of the external shock. Using a Fokker-Planck approach to model the evolution of protons and electrons, and the non-linear feedback of turbulence damping, we show that the inverse Compton radiation of the second electron component may harden the observed spectrum above multi-TeV energy, and significantly ameliorate the fitting to the spectral evolution measured by LHAASO without violating lower-energy observations. We also discuss the potential presence of the second electron component in other GRB afterglows, which may provide a possible observational signature for future studies.

astro-ph.HE↗

arXiv2023

Particle transport through localized interactions with sharp magnetic field bends in MHD turbulence

When a particle crosses a region of space where the curvature radius of the magnetic field line shrinks below its gyroradius $r_{\rm g}$, it experiences a non-adiabatic (magnetic moment violating) change in pitch angle. The present paper carries that observation into magnetohydrodynamic (MHD) turbulence to examine the influence of intermittent, sharp bends of the magnetic field lines on particle transport. On the basis of dedicated measurements in a simulation of incompressible turbulence, it is argued that regions of sufficiently large curvature exist in sufficient numbers on all scales to promote scattering. The parallel mean free path predicted by the power-law statistics of the curvature strength scales in proportion to $r_{\rm g}^{0.3}\,\ell_{\rm c}^{0.7}$ ($\ell_{\rm c}$ coherence scale of the turbulence), which is of direct interest for cosmic-ray phenomenology. Particle tracking in that numerical simulation confirms that the magnetic moment diffuses through localized, violent interactions, in agreement with the above picture. Correspondingly, the overall transport process is non-Brownian up to length scales $\gtrsim\ell_{\rm c}$.

physics.plasm-ph↗

arXiv2023

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 $γ_{\rm sh} \simeq 2.7$, magnetization level $σ\simeq 0.01$), 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 $\mathrm{d}N/\mathrm{d}γ\propto γ^{-s}$ 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.

astro-ph.HE↗

arXiv2023

Particle Acceleration in Colliding Flows: Binary Star Winds and Other Double-Shock Structures

A shock wave propagating perpendicularly to an ambient magnetic field accelerates particles considerably faster than in the parallel propagation regime. However, the perpendicular acceleration stops after the shock overruns a circular particle orbit. At the same time, it may continue in flows resulting from supersonically colliding plasmas bound by a pair of perpendicular shocks. Although the double-shock acceleration mechanism, which we consider in detail, is not advantageous for thermal particles, pre-energized particles may avoid the premature end of acceleration. We argue that if their gyroradius exceeds the dominant turbulence scale between the shocks, these particles might traverse the intershock space repeatedly before being carried away by the shocked plasma. Moreover, entering the space between the shocks of similar velocities $u_{1}\approx u_{2}\approx c$, such particles start bouncing between the shocks at a fixed angle $\approx 35.3^{\circ}$ to the shock surface. Their drift along the shock fronts is slow, $V_{d}\sim\left|u_{2}-u_{1}\right|\ll c$, so that it will take $N\sim Lc/\left|u_{2}-u_{1}\right|d\gg1$ bounces before they escape the accelerator (here, $L$ is the size of the shocks and $d$ is the gap between them). Since these particles more than ten-fold their energy per cycle (two consecutive bounces), we invoke other possible losses that can limit the acceleration. They include drifts due to rippled shocks, the nonparallel mutual orientation of the upstream magnetic fields, and radiative losses.

astro-ph.HE↗

arXiv2022

First-principles Fermi acceleration in magnetized turbulence

This work provides a concrete implementation of E. Fermi's model of particle acceleration in magnetohydrodynamic (MHD) turbulence, connecting the rate of energization to the gradients of the velocity of magnetic field lines, which it characterizes within a multifractal picture of turbulence intermittency. It then derives a transport equation in momentum space for the distribution function. This description is shown to be substantiated by a large-scale numerical simulation of strong MHD turbulence. The present, general framework can be used to model particle acceleration in a variety of environments.

astro-ph.HE↗

arXiv2022

Non-resonant particle acceleration in strong turbulence: comparison to kinetic and MHD simulations

Collisionless, magnetized turbulence offers a promising framework for the generation of non-thermal 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 PIC, as well as 3D (incompressible) magnetohydrodynamic (MHD) simulations, we test here a recent model of non-resonant particle acceleration in strongly magnetized turbulence~\cite{2021PhRvD.104f3020L}, 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 non-resonant 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 PIC simulations, while in the MHD incompressible simulation, both the parallel shear and the transverse compressive term provide about equal contributions.

astro-ph.HE↗

arXiv2022

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.

astro-ph.HE↗

arXiv2021

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

The 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.

physics.plasm-ph↗

arXiv2021

Particle acceleration in strong MHD turbulence

Nonthermal acceleration of particles in magnetohydrodynamic (MHD) turbulence plays a central role in a wide variety of astrophysical sites. This physics is addressed here in the context of a strong turbulence, composed of coherent structures rather than waves, beyond the realm of quasilinear theory. The present description tracks the momentum of the particle through a sequence of frames in which the electric field vanishes, in the spirit of the original Fermi scenario. It connects the sources of energy gain (or loss) to the gradients of the velocity of the magnetic field lines, in particular the acceleration and the shear of their velocity flow projected along the field line direction, as well as their compression in the transverse plane. Those velocity gradients are subject to strong intermittency: they are spatially localized and their strengths obey power law distributions, as demonstrated through direct measurements in the incompressible MHD simulation of the Johns Hopkins University database. This intermittency impacts the acceleration process in a significant way, which opens up prospects for a rich phenomenology. In particular, the momentum distribution, which is here captured through an analytical random walk model, displays extended power law tails with soft-to-hard evolution in time, in general agreement with recent kinetic numerical simulations. Extensions to this description and possible avenues of exploration are discussed.

astro-ph.HE↗

arXiv2021

Electron-proton co-acceleration on relativistic shocks in extreme-TeV blazars

The multi-wavelength emission from a newly identified population of `extreme-TeV' blazars, with Compton peak frequencies around 1 TeV, is difficult to interpret with standard one-zone emission models. Large values of the minimum electron Lorentz factor and quite low magnetisation values seem to be required. We propose a scenario where protons and electrons are co-accelerated on internal or recollimation shocks inside the relativistic jet. In this situation, energy is transferred from the protons to the electrons in the shock transition layer, leading naturally to a high minimum Lorentz factor for the latter. A low magnetisation favours the acceleration of particles in relativistic shocks. The shock co-acceleration scenario provides additional constraints on the set of parameters of a standard one-zone lepto-hadronic emission model, reducing its degeneracy. Values of the magnetic field strength of a few mG and minimum electron Lorentz factors of 10^3 to 10^4, required to provide a satisfactory description of the observed spectral energy distributions of extreme blazars, result here from first principles. While acceleration on a single standing shock is sufficient to reproduce the emission of most of the extreme-TeV sources we have examined, re-acceleration on a second shock appears needed for those objects with the hardest gamma-ray spectra. Emission from the accelerated proton population, with the same number density as the electrons but in a lower range of Lorentz factors, is strongly suppressed. Satisfactory self-consistent representations were found for the most prominent representatives of this new blazar class.

astro-ph.HE↗

arXiv2020

Powerlaw spectra from stochastic acceleration

Numerical simulations of particle acceleration in magnetized turbulence have recently observed powerlaw spectra where pile-up distributions are rather expected. We interpret this as evidence for particle segregation based on acceleration rate, which is likely related to a non-trivial dependence of the efficacy of acceleration on phase space variables other than the momentum. We describe the corresponding transport in momentum space using continuous-time random walks, in which the time between two consecutive momentum jumps becomes a random variable. We show that powerlaws indeed emerge when the experimental (simulation) timescale does not encompass the full extent of the distribution of waiting times. We provide analytical solutions, which reproduce dedicated numerical Monte Carlo realizations of the stochastic process, as well as analytical approximations. Our results can be readily extrapolated for applications to astrophysical phenomenology.

astro-ph.HE↗

arXiv2020

Generalized Fermi acceleration

In highly conducting astrophysical plasmas, charged particles are generically accelerated through Fermi-type processes involving repeated interactions with moving magnetized scattering centers. The present paper proposes a generalized description of these acceleration processes, by following the momentum of the particle through a continuous sequence of accelerated frames, defined in such a way that the electric field vanishes at each point along the particle trajectory. In each locally inertial frame, the Lorentz force affects the direction of motion of the particle, but the energy changes solely as a result of inertial corrections. This unified description of Fermi acceleration applies equally well in sub- and ultrarelativistic settings, in Cartesian or non-Cartesian geometries, flat or nonflat space-time. Known results are recovered in a variety of regimes -- shock, turbulent and shear acceleration -- and new results are derived in lieu of applications, e.g. nonresonant acceleration in relativistic turbulence, stochastic unipolar inductive acceleration and centrifugo-shear acceleration close to the horizon of a black hole.

astro-ph.HE↗

arXiv2020

Progress in unveiling extreme particle acceleration in persistent astrophysical jets

The most powerful persistent accelerators in the Universe are jetted active galaxies. Blazars, galaxies whose jets are directed towards Earth, dominate the extragalactic gamma-ray sky. Still, most of the highest-energy particle accelerators likely elude detection. These extreme blazars, whose radiated energy can peak beyond 10 TeV, are ideal targets to study particle acceleration and radiative processes, and may provide links to cosmic rays and astrophysical neutrinos. The growing number of extreme blazars observed at TeV energies has been critical for the emergence of gamma-ray cosmology, including measurements of the extragalactic background light, tight bounds on the intergalactic magnetic field, and constraints on exotic physics at energies inaccessible with human-made accelerators. Tremendous progress has been achieved over the past decade, which bodes well for the future, particularly with the deployment of the Cherenkov Telescope Array.

astro-ph.HE↗

arXiv2020

Particle acceleration in relativistic turbulence: A theoretical appraisal

We discuss the physics of stochastic particle acceleration in relativistic magnetohydrodynamic (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 wavelike isotropic fast mode turbulence, in Alfvén and slow Goldreich-Sridhar type wave turbulence (properly accounting for 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 Alfvén 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 nonresonant 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.

astro-ph.HE↗

arXiv2019

Physics of relativistic collisionless shocks: III The suprathermal particles

In this third paper of a series, we discuss the physics of the population of accelerated particles in the precursor of an unmagnetized, relativistic collisionless pair shock. In particular, we provide a theoretical estimate of their scattering length $l_{scatt}(p)$ in the self-generated electromagnetic turbulence, as well as an estimate of their distribution function. We obtain $l_{scatt}(p) \simeq (γ_p /ε_B)(p/γ_{\infty} mc)^2 (c/ω_p)$, with p the particle momentum in the rest frame of the shock front, $ε_B$ the strength parameter of the microturbulence, $γ_p$ the Lorentz factor of the background plasma relative to the shock front and $γ_{\infty}$ its asymptotic value outside the precursor. We compare this scattering length to large-scale PIC simulations and find good agreement for the various dependencies.

astro-ph.HE↗

arXiv2018

Stability analysis of a periodic system of relativistic current filaments

The 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 instabilites in symmetric two-beam systems.

physics.plasm-ph↗