arXiv subjects

Omar French

Publications and source records attributed to Omar French.

7 recordsLinked to original sources

arXiv2026

Can magnetic reconnection power neutrino emission from AGN coronae?

We investigate whether reconnection of small-scale current sheets in transrelativistic supermassive black hole (SMBH) coronae can supply the nonthermal protons needed for high-energy neutrino emission, using NGC 1068 as a test case. We model the corona as a strongly turbulent, low-$\beta$, collisionless hydrogen plasma with characteristic size $r_{\rm co}$, magnetic field strength $B$, proton density $n_p$, and radiation energy density $u_{\rm rad}$. Combining the observed IceCube-band neutrino luminosity with the X-ray luminosity and Thomson optical depth reduces these coronal quantities to a one-parameter family. Across this family, the proton magnetization $\sigma_p \equiv B^2/(4\pi n_p m_p c^2)$ is transrelativistic with $\sigma_p \sim 0.3$. In this regime, we show that repeated encounters with intermittent reconnecting current sheets can energize suprathermal protons up to tens of PeV before photomeson cooling limits further acceleration. These injected particles may then be further processed by stochastic interactions with the turbulent cascade. Motivated by PIC simulations of strong turbulence at comparable magnetization, we adopt a nonthermal proton spectrum with an independently specified index and find that the predicted TeV spectral shape is broadly consistent with NGC~1068 without fitting the proton spectral slope.

astro-ph.HE↗

arXiv2025

Particle injection in three-dimensional relativistic magnetic reconnection

Relativistic magnetic reconnection has been proposed as an important nonthermal particle acceleration (NTPA) mechanism that generates power-law spectra and high-energy emissions. Power-law particle spectra are in general characterized by three parameters: the power-law index, the high-energy cutoff, and the low-energy cutoff (i.e., the injection energy). Particle injection into the nonthermal power law, despite also being a critical step in the NTPA chain, has received considerably less attention than the subsequent acceleration to high energies. Open questions on particle injection that are important for both physical understanding and astronomical observations include how the upstream magnetization~$\sigma$ influences the injection energy and the contributions of the known injection mechanisms (i.e., direct acceleration by the reconnection electric field, Fermi kicks, and pickup acceleration) to the injected particle population. Using fully kinetic particle-in-cell simulations, we uncover these relationships by systematically measuring the injection energy and calculating the contributions of each acceleration mechanism to the total injected particle population. We also present a theoretical model to explain these results. Additionally, we compare two- and three-dimensional simulations to assess the impact of the flux-rope kink and drift-kink instability on particle injection. We conclude with comparisons with previous work and outlook for future work.

astro-ph.HE↗

arXiv2025

Particle Injection Problem in Magnetic Reconnection and Turbulence

Magnetic reconnection and turbulence in magnetically-dominated environments have been proposed as important nonthermal particle acceleration mechanisms that generate high energy particles and associated emissions. While the acceleration to high energy that produces the power-law energy distribution has drawn strong interest, recent studies actively discuss pre-acceleration, or injection, to a sufficient energy for a sustained and prolonged Fermi-like acceleration. The injection process is important for determining the fraction of nonthermal particles and energy partition between thermal and nonthermal particles. We review recent advances in understanding the injection mechanisms responsible for populating these nonthermal power-law spectra, and conclude with an outlook for studies and applications of injection models.

physics.plasm-ph↗

arXiv2024

Low-energy Injection and Nonthermal Particle Acceleration in Relativistic Magnetic Turbulence

Relativistic magnetic turbulence has been proposed as a process for producing nonthermal particles in high-energy astrophysics. Particle energization may be contributed by both magnetic reconnection and turbulent fluctuations, but their interplay is poorly understood. It has been suggested that during magnetic reconnection the parallel electric field dominates particle acceleration up to the lower bound of the power-law particle spectrum, but recent studies show that electric fields perpendicular to magnetic field can play an important, if not dominant role. In this study, we carry out 2D fully kinetic particle-in-cell simulations of magnetically dominated decaying turbulence in a relativistic pair plasma. For a fixed magnetization parameter $\sigma_0=20$, we find that the injection energy ${\varepsilon}_{\rm inj}$ converges with increasing domain size to ${\varepsilon}_{\rm inj}\simeq 10m_ec^2$. In contrast, the power-law index, the cut-off energy, and the power-law extent increase steadily with domain size. We trace a large number of particles and evaluate the contributions of the work done by the parallel ($W_\parallel$) and perpendicular ($W_\perp$) electric fields during both the injection phase and the post-injection phase. We find that during the injection phase, the $W_\perp$ contribution increases with domain size, suggesting that it may eventually dominate injection for a sufficiently large domain. In contrast, both components contribute equally during the post-injection phase, insensitive to the domain size. For high energy (${\varepsilon}\gg{\varepsilon}_{\rm inj}$) particles, $W_\perp$ dominates the subsequent energization. These findings may improve our understanding of nonthermal particles and their emissions in astrophysical plasmas.

astro-ph.HE↗

arXiv2023

Particle Injection and Nonthermal Particle Acceleration in Relativistic Magnetic Reconnection

Magnetic reconnection in the relativistic regime has been proposed as an important process for the efficient production of nonthermal particles and high-energy emissions. Using fully kinetic particle-in-cell simulations, we investigate how guide-field strength and domain size affect characteristic spectral features and acceleration processes. We study two stages of acceleration: energization up until the injection energy $γ_{\rm inj}$ and further acceleration that generates a power-law spectrum. Stronger guide fields increase the power-law index and $γ_{\rm inj}$, which suppresses acceleration efficiency. These quantities seemingly converge with increasing domain size, suggesting that our findings can be extended to large-scale systems. We find that three distinct mechanisms contribute to acceleration during injection: particle streaming along the parallel electric field, Fermi reflection, and the pickup process. Fermi and pickup processes, related to the electric field perpendicular to the magnetic field, govern the injection for weak guide fields and larger domains. Meanwhile, parallel electric fields are important for injection in the strong guide field regime. In the post-injection stage, we find that perpendicular electric fields dominate particle acceleration in the weak guide field regime, whereas parallel electric fields control acceleration for strong guide fields. These findings will help explain the nonthermal acceleration and emissions in high-energy astrophysics, including black hole jets and pulsar wind nebulae.

astro-ph.HE↗

arXiv2022

Comment on "Nonideal Fields Solve the Injection Problem in Relativistic Reconnection"

Recently, Sironi (PRL, 128, 145102; S22) reported the correlation between particles accelerated into high energy and their crossings of regions with electric field larger than magnetic field (E>B regions) in kinetic simulations of relativistic magnetic reconnection. They claim that electric fields in E>B regions (for a vanishing guide field) dominate in accelerating particles to the injection energy. S22 presented test-particle simulations showing that if particle energies are reset to low energies in E>B regions, efficient injection is suppressed. This Comment re-examines these claims by analyzing a simulation resembling the reference case in S22. We show that during crossings E>B acceleration only contributes a small fraction to the injection energy as E>B regions only host particles for a short duration. The energization before any E>B crossings has a comparable contribution, indicating E>B regions are not unique in pre-accelerating particles. A new test-particle simulation shows that zero-outing electric fields in E>B regions does not strongly influence the injection. We suggest that the procedure used in S22 to exclude E>B acceleration partly removes acceleration outside E>B regions, leading to a false conclusion.

astro-ph.HE↗

arXiv2022

Direct measurement of non-thermal electron acceleration from magnetically driven reconnection in a laboratory plasma

Magnetic reconnection is a ubiquitous astrophysical process that rapidly converts magnetic energy into some combination of plasma flow energy, thermal energy, and non-thermal energetic particles, including energetic electrons. Various reconnection acceleration mechanisms in different low-$β$ (plasma-to-magnetic pressure ratio) and collisionless environments have been proposed theoretically and studied numerically, including first- and second-order Fermi acceleration, betatron acceleration, parallel electric field acceleration along magnetic fields, and direct acceleration by the reconnection electric field. However, none of them have been heretofore confirmed experimentally, as the direct observation of non-thermal particle acceleration in laboratory experiments has been difficult due to short Debye lengths for \textit{in-situ} measurements and short mean free paths for \textit{ex-situ} measurements. Here we report the direct measurement of accelerated non-thermal electrons from low-$β$ magnetically driven reconnection in experiments using a laser-powered capacitor coil platform. We use kiloJoule lasers to drive parallel currents to reconnect MegaGauss-level magnetic fields in a quasi-axisymmetric geometry. The angular dependence of the measured electron energy spectrum and the resulting accelerated energies, supported by particle-in-cell simulations, indicate that the mechanism of direct electric field acceleration by the out-of-plane reconnection electric field is at work. Scaled energies using this mechanism show direct relevance to astrophysical observations. Our results therefore validate one of the proposed acceleration mechanisms by reconnection, and establish a new approach to study reconnection particle acceleration with laboratory experiments in relevant regimes.

physics.plasm-ph↗