arXiv subjects

Daniel Humphrey

Publications and source records attributed to Daniel Humphrey.

4 recordsLinked to original sources

arXiv2026

Particle acceleration in Alfv\'enic turbulence with a strong guide field

Magnetically dominated Alfv\'enic turbulence creates an effective environment for particle acceleration. However, when a strong mean field is present, traditional mechanisms like mirror and curvature acceleration become inefficient at explaining non-thermal particle energy distributions. Based on numerical and phenomenological study, we propose that in such turbulence, particles are accelerated in charge-starved current sheets, corresponding to current velocities approaching the speed of light. The distributions of the electric currents, plasma density, fluctuations of electric charge, as well as the energy distributions of accelerated particles, approximately follow log-normal statistics. Non-thermal particle distributions thus arise from particle acceleration in these charge-starved current sheets rather than from conventional Fermi-type particle acceleration by turbulent eddies.

astro-ph.HE↗

arXiv2026

Distributions of particles accelerated by strong Alfv\'enic turbulence

This work presents a model for generating nonthermal power-law tails of particles' energy probability density functions in turbulent collisionless plasmas, applicable to both non-relativistic and relativistic scenarios. We propose that strong Alfv\'enic turbulence energizes plasma particles through curvature acceleration, particularly for particles with Larmor radii comparable to the scales of turbulence. When the energy density of the energized particles increases, the efficiency of the energy exchange process diminishes. As a result, the acceleration process saturates, leading to power-law distributions of particle momentum and energy. In the non-relativistic case, the momentum probability density function scales as $f(p) dp \propto p^{-3} dp $, while in the ultrarelativistic case, the energy probability density function scales as $ f(\gamma) d\gamma \propto \gamma^{-3} d\gamma $, where $\gamma$ is the Lorentz factor. This model provides a unified framework for understanding particle acceleration in both energy regimes, complementing existing analytical approaches. The predicted scalings are consistent with available observations of energetic ion distributions in the heliosphere and with the findings from numerical simulations of ultrarelativistic particle acceleration in magnetically dominated plasma turbulence.

physics.plasm-ph↗

arXiv2026

Particle acceleration and pitch-angle evolution in relativistic turbulence

Synchrotron radiation detected from relativistic astrophysical objects such as pulsar-wind nebulae and {jets from active galactic nuclei} depends on the magnetic fields and the distribution functions of energetic electrons in these systems. Relativistic magnetically dominated turbulence has been recognized as an efficient mechanism for structure formation and non-thermal particle acceleration in these environments. Recent numerical simulations of relativistic turbulence have provided insights into the energy distribution functions of accelerated electrons. Much less is currently understood about their {pitch angle distributions}, which are crucial for accurately interpreting the spectra of synchrotron radiation. {We perform a detailed case study of} the pitch angle distributions formed during the process of turbulent acceleration {for $B_0/\delta B_0 = 10$ and $\tilde{\sigma}_0 \sim 40$, where $B_0$ is the uniform component of the magnetic field, $\delta B_0$ is the fluctuating component, and $\tilde{\sigma}_0$ is the plasma magnetization based on the magnetic fluctuations. We find that even minimal numerical noise can cause substantial pitch angle scattering, but we demonstrate techniques for overcoming the numerical challenges associated with the evolution of very small pitch angles. Our numerical results are consistent with the phenomenological model found in \cite[][]{vega2024b,vega2025}.}

astro-ph.GA↗

arXiv2025

Modeling the Reverberation Response of the Broad Line Region in Active Galactic Nuclei II: Incorporating Photoionization Models

The broad emission lines (BELs) emitted by Active Galactic Nuclei respond to variations in the ionizing continuum emission from the accretion disk surrounding the central supermassive black hole (SMBH). This reverberation response provides insights into the structure and dynamics of the Broad Line Region (BLR). In Rosborough et al., 2024, we introduced a new forward-modeling tool, the Broad Emission Line MApping Code (BELMAC), which simulates the velocity-resolved reverberation response of the BLR to an input light curve. In this work, we describe a new version of BELMAC, which uses photoionization models to calculate the cloud luminosities for selected BELs. We investigated the reverberation responses of H$\alpha$, H$\beta$, MgII$\lambda$2800 and CIV$\lambda$1550 for models representing a disk-like BLR with Keplerian rotation, radiatively driven outflows, and inflows. The line responses generally provide a good indication of the respective luminosity-weighted radii. However, there are situations when the BLR exhibits a negative response to the driving continuum, causing overestimates of the luminosity-weighted radius. The virial mass derived from the models can differ dramatically from the actual SMBH mass, depending mainly on the disk inclination and velocity field. In single zone models, the BELs exhibit similar responses and profile shapes; two-zone models, such as a Keplerian disk and a biconical outflow, can reproduce observed differences between high- and low-ionization lines. Radial flows produce asymmetric line profile shapes due to both anisotropic cloud emission and electron scattering in an inter-cloud medium. These competing attenuation effects complicate the interpretation of profile asymmetries.

astro-ph.GA↗