arXiv subjects

Jens F. Mahlmann

Publications and source records attributed to Jens F. Mahlmann.

10 recordsLinked to original sources

arXiv2026

1D PIC Simulations of Resonant Scattering-Driven Pair Cascades in Magnetar Magnetospheres

Hard X-rays in persistent magnetar emission originate from radiative processes in pair-loaded magnetospheric plasma, yet self-consistent kinetic simulations of such radiation-rich systems remain limited. We perform 1D particle-in-cell simulations of pair creation mediated by resonant inverse Compton scattering (RICS) along isolated field lines in twisted magnetospheres, capturing particle acceleration and radiative drag from first principles. The drag force, which strongly depends on the particle Lorentz factor and location, plays a key role in the magnetospheric circuit. Strong RICS drag impedes the flow of electrons and positrons, particularly along extended field lines. Nevertheless, the plasma sustains the circuit by self-organizing into a single accelerating gap near the star in one hemisphere. The accelerated plasma flow from the gap becomes loaded with copious electron-positron pairs in extended RICS zones, and also carries ions extracted from the star and accelerated in the gap. When radiative drag stops the pair-loaded flow at the magnetic equator, the ion component transfers momentum through streaming instabilities, delivering a small lepton population into the opposite hemisphere and sustaining the circuit. The gap confined to a single (anode) hemisphere implies asymmetric hard X-ray production, possibly detectable in phase-resolved magnetar spectra.

astro-ph.HE↗

arXiv2026

Linear Mode Conversion in Ultramagnetized Pair Plasmas: Single-Parameter Scaling

In neutron star (NS) magnetospheres, plasma waves propagate as normal modes with distinct propagation dynamics that strongly influence observable signals. This letter presents a unified theory of linear mode conversion between Alfv'en (A), superluminal ordinary (O), and extraordinary (X) modes, incorporating the effect of magnetic-field geometry and local plasma response. Magnetic field-line curvature induces A-X conversion for low frequencies and O-X conversion at high frequencies, whereas plasma gradients alone do not drive X-mode coupling. We show that a single dimensionless parameter controls both conversion channels. The conversion efficiency follows the universal nonadiabatic transition probability of a multilevel quantum system. Efficient conversion occurs within a narrow angular window between the wave vector and magnetic field, localizing potential conversion sites in the NS magnetosphere. This linear mechanism naturally accounts for complex polarization features observed in pulsars and some fast radio bursts.

astro-ph.HE↗

arXiv2026

Superluminal Wave Activation at Relativistic Magnetized Shocks

Fast radio bursts (FRBs) are extremely energetic radio transients, some are generated in magnetar magnetospheres and winds. Despite a growing number of observations, their emission mechanisms remain elusive. It has recently been proposed that Alfvénic perturbations can convert into superluminal O-modes at magnetized shocks and propagate downstream as a radio signal. We validate this superluminal wave activation mechanism using pair-plasma theory and particle-in-cell simulations. Theory predicts two different downstream modes: nonpropagating Alfvénic perturbations and propagating superluminal O-modes. Superluminal wave activation occurs if the frequency of upstream perturbations in the shock frame exceeds the downstream plasma frequency. 1D particle-in-cell simulations confirm wavenumber and frequency jumps across the shock for upstream perturbations with frequencies well above the plasma frequency. Our simulations model both monochromatic upstream waves and broadband spectra with the downstream plasma frequency acting like a high-pass filter for superluminal O-modes. We discuss implications for FRB generation in relativistic magnetized winds.

astro-ph.HE↗

arXiv2025

Entity -- Hardware-agnostic Particle-in-Cell Code for Plasma Astrophysics. I: Curvilinear Special Relativistic Module

Entity is a new-generation, fully open-source particle-in-cell (PIC) code developed to overcome key limitations in astrophysical plasma modeling, particularly the extreme separation of scales and the performance challenges associated with evolving, GPU-centric computing infrastructures. It achieves hardware-agnostic performance portability across various GPU and CPU architectures using the Kokkos library. Crucially, Entity maintains a high standard for usability, clarity, and customizability, offering a robust and easy-to-use framework for developing new algorithms and grid geometries, which allows extensive control without requiring edits to the core source code. This paper details the core general-coordinate special-relativistic module. Entity is the first PIC code designed to solve the Vlasov-Maxwell system in general coordinates, enabling a coordinate-agnostic framework that provides the foundational structure for straightforward extension to arbitrary coordinate geometries. The core methodology achieves numerical stability by solving particle equations of motion in the global orthonormal Cartesian basis, despite using generalized coordinates like Cartesian, axisymmetric spherical, and quasi-spherical grids. Charge conservation is ensured via a specialized current deposition technique using conformal currents. The code exhibits robust scalability and performance portability on major GPU platforms (AMD MI250X, NVIDIA A100, and Intel Max Series), with the 3D particle pusher and the current deposition operating efficiently at about 2 nanoseconds per particle per timestep. Functionality is validated through a comprehensive suite of standard Cartesian plasma tests and the accurate modeling of relativistic magnetospheres in curvilinear axisymmetric geometries.

astro-ph.HE↗

arXiv2025

Electrodynamics and dissipation in the binary magnetosphere of pre-merger neutron stars

We investigate energy release in the interacting magnetospheres of binary neutron stars (BNSs) with global 3D force-free electrodynamics simulations. The system dynamics depend on the inclinations $χ_1$ and $χ_2$ of the stars' magnetic dipole moments relative to their orbital angular momentum. The simplest aligned configuration ($χ_1=χ_2=0^\circ$) has no magnetic field lines connecting the two stars. Remarkably, it still develops separatrix current sheets warping around each star and a dissipative region at the interface of the two magnetospheres. A Kelvin-Helmholtz (KH)-type instability drives significant dissipation at the magnetospheric interface, generating local Alfvénic turbulence and escaping fast magnetosonic waves. Binaries with inclined magnetospheres release energy in two ways: via KH instability at the interface and via magnetic reconnection flares in the twisted flux bundles connecting the companions. Outgoing compressive waves occur in a broad range of BNS parameters, possibly developing shocks and sourcing fast radio bursts. We discuss implications for X-ray and radio precursors of BNS mergers.

astro-ph.HE↗

arXiv2024

Force-free wave interaction in magnetar magnetospheres: Computational modeling in axisymmetry

Crustal quakes of highly magnetized neutron stars can disrupt their magnetospheres, triggering energetic phenomena like X-ray and fast radio bursts (FRBs). Understanding plasma wave dynamics in these extreme environments is vital for predicting energy transport across scales to the radiation length. This study models relativistic plasma wave interaction in magnetar magnetospheres with force-free electrodynamics simulations. For propagation along curved magnetic field lines, we observe the continuous conversion of Alfvén waves to fast magnetosonic (FMS) waves. The conversion efficiency can be up to three times higher when counter-propagating Alfvén waves interact in the equatorial region. Alfvén waves generate FMS waves of twice their frequency during their first crossing of the magnetosphere. After the initial transient burst of FMS waves, Alfvén waves convert to FMS waves periodically, generating variations on timescales of the magnetospheric Alfvén wave crossing time. This decaying FMS wave tail carries a significant portion (half) of the total energy emitted. Plastic damping of 'bouncing' Alfvén waves by the magnetar crust has minimal impact on the FMS efficiency. We discuss the implications of the identified wave phenomena for magnetar observations. Outgoing FMS waves can develop electric zones, potential sources of coherent radiation. Long-wavelength FMS waves could generate FRBs through reconnection beyond the light cylinder.

astro-ph.HE↗

arXiv2024

Spindown of Pulsars Interacting with Companion Winds: Impact of Magnetospheric Compression

The presence of a companion wind in neutron star binary systems can form a contact discontinuity well within the pulsar's light cylinder, effectively creating a waveguide that confines the pulsar's electromagnetic fields and significantly alters its spindown. We parametrize this confinement as the ratio between the equatorial position of the contact discontinuity (or standoff distance) $R_\mathrm{m}$ and the pulsar's light cylinder $R_\mathrm{LC}$. We quantify the pulsar spindown for relativistic wind envelopes with $R_\mathrm{m}/R_\mathrm{LC} = 1/3...1$ and varying inclination angles $χ$ between magnetic and rotation axes using particle-in-cell simulations. Our strongly confined models ($R_\mathrm{m}/R_\mathrm{LC} = 1/3$) identify two distinct limits. For $χ=0^\circ$, the spindown induced by the compressed pulsar magnetosphere is enhanced by approximately three times compared to an isolated pulsar due to an increased number of open magnetic field lines. Conversely, for $χ=90^\circ$, the compressed system spins down at less than $40\%$ of the rate of an isolated reference pulsar due to the mismatch between the pulsar wind stripe wavelength and the waveguide size. We directly apply our analysis to the 2.77-second period oblique rotator ($χ=60^\circ$) in the double pulsar system PSR J0737-3039. With the numerically derived spindown estimate, we constrain its surface magnetic field to $B_* \approx (7.3 \pm 0.2) \times 10^{11}$ G. We discuss the time modulation of its period derivative, the effects of compression on its braking index, and implications for the radio eclipse in PSR J0737-3039.

astro-ph.HE↗

arXiv2024

Safety first: Stability and dissipation of line-tied force-free flux tubes in magnetized coronae

Magnetized plasma columns and extended magnetic structures with both footpoints anchored to a surface layer are an important building block of astrophysical dissipation models. Current loops shining in X-rays during the growth of plasma instabilities are observed in the corona of the Sun and are expected to exist in highly magnetized neutron star magnetospheres and accretion disk coronae. For varying twist and system sizes, we investigate the stability of line-tied force-free flux tubes and the dissipation of twist energy during instabilities using linear analysis and time-dependent force-free electrodynamics simulations. Kink modes ($m=1$) and efficient magnetic energy dissipation develop for plasma safety factors $q\lesssim 1$, where $q$ is the inverse of the number of magnetic field line windings per column length. Higher-order fluting modes ($m>1$) can distort equilibrium flux tubes for $q>1$ but induce significantly less dissipation. In our analysis, the characteristic pitch $\tildeμ_0$ of flux tube field lines determines the growth rate ($\propto \tildeμ_0^3$) and minimum wavelength of the kink instability ($\propto \tildeμ_0^{-1}$). We use these scalings to determine a minimum flux tube length for the growth of the kink instability for any given $\tildeμ_0$. By drawing analogies to idealized magnetar magnetospheres with varying regimes of boundary shearing rates, we discuss the expected impact of the pitch-dependent growth rates for magnetospheric dissipation in magnetar conditions.

astro-ph.SR↗

arXiv2022

Propagation of a Strong Fast Magnetosonic Wave in the Magnetosphere of a Neutron Star

We study the propagation of a strong, low frequency, linearly polarized fast magnetosonic wave inside the magnetosphere of a neutron star. The relative strength $δB/B$ of the wave grows as a function of radius before it reaches the light cylinder, and what starts as a small perturbation can grow to become nonlinear before it escapes the magnetosphere. Using first-principles Particle-in-Cell (PIC) simulations, we study in detail the evolution of the wave as it becomes nonlinear. We find that an initially sinusoidal wave becomes strongly distorted as $δB/B$ approaches order unity. The wave steepens into a shock in each wavelength. The plasma particles drift into the shock and undergo coherent gyration in the rest of the wave, and subsequently become thermalized. This process quickly dissipates the energy of an FRB emitted deep within the magnetosphere of magnetar, effectively preventing GHz waves produced in the closed field line zone from escaping. This mechanism may also provide an effective way to launch shocks in the magnetosphere from kHz fast magnetosonic waves without requiring a relativistic ejecta. The resulting shock can propagate to large distances and may produce FRBs as a synchrotron maser.

astro-ph.HE↗

arXiv2022

The role of plasma instabilities in relativistic radiation mediated shocks: stability analysis and particle-in-cell simulations

Relativistic radiation mediated shocks (RRMS) likely form in prodigious cosmic explosions. The structure and emission of such shocks is regulated by copious production of electron-positron pairs inside the shock transition layer. It has been pointed out recently that substantial abundance of positrons inside the shock leads to a velocity separation of the different plasma constituents, which is expected to induce a rapid growth of plasma instabilities. In this paper, we study the hierarchy of plasma microinstabilities growing in an electron-ion plasma loaded with pairs and subject to a radiation force. Linear stability analysis indicates that such a system is unstable to the growth of various plasma modes which ultimately become dominated by a current filamentation instability driven by the relative drift between the ions and the pairs. These results are validated by particle-in-cell simulations that further probe the nonlinear regime of the instabilities, and the pair-ion coupling in the microturbulent electromagnetic field. Based on this analysis, we derive a reduced transport equation for the particles via pitch angle scattering in the microturbulence and demonstrate that it can couple the different species and lead to nonadiabatic compression via a Joule-like heating. The heating of the pairs and, conceivably, the formation of nonthermal distributions, arising from the microturbulence, can affect the observed shock breakout signal in ways unaccounted for by current single-fluid models.

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