arXiv subjects

Anatoly Spitkovsky

Publications and source records attributed to Anatoly Spitkovsky.

At least 19 recordsLinked to original sources

arXiv2026

Synchrotron-Regulated Relativistic Magnetohydrodynamic Turbulence: Emission, Polarization, and Faraday Rotation

Relativistic magnetized plasmas in many high-energy astrophysical systems are both turbulent and strongly radiative, yet their nonlinear dynamics and radiative outcomes remain poorly understood. Here we present results from three-dimensional driven turbulence simulations in relativistic magnetohydrodynamics with synchrotron cooling. We compute Faraday rotation measures, synthetic synchrotron spectra and linear polarization maps from the simulated turbulence. The balance between energy injection from turbulent driving and synchrotron cooling keeps the plasma, on average, relativistically hot, thereby influencing the rotation measure. Synchrotron cooling triggers the thermal instability and drives the plasma into hot dilute and cold dense phases, which enhances the spatial and temporal variability of synchrotron emission, especially at high frequencies. These diagnostics show qualitative similarities to observations of fast radio bursts, pulsar wind nebulae, and blazars, suggesting that turbulence may play an important role in shaping emission and propagation effects around high-energy sources.

astro-ph.HE↗

arXiv2026

Diocotron Modes in Pulsar Magnetospheres: Charge Diffusion and Implications for Radio Emission Variability

The diocotron instability is a non-axisymmetric plasma instability that should occur generically in the differentially rotating equatorial plane of pulsar magnetospheres. We present a series of 3D particle-in-cell (PIC) simulations of the diocotron instability in aligned and oblique pulsars. The instability grows on timescales of the rotation period and develops a strong, stable $m=1$ mode, corresponding to a rotating, dipolar charge asymmetry in the equatorial disk. Stochastic fluctuations in the diocotron mode amplitude and pattern speed drive cross-field diffusion that can rapidly transport charges through the closed zone toward the light cylinder. In the nonlinear stage, the $m=1$ mode produces electric field perturbations which can modulate the polar cap potential drop and the emission beam angle, with possible connections to pulsar variability such as nulling, periodic amplitude modulation, and drifting subpulses.

astro-ph.HE↗

arXiv2026

Electromagnetic Precursors to Binary Neutron Star Mergers: Kinetic Simulations of Magnetospheric Flaring

We present the first 3D global kinetic simulations of the interacting magnetospheres of pre-merger binary neutron stars. The stars, whose magnetic moments are anti-aligned, twist the field lines connecting them, leading to periodic eruptions. Each eruption consists of an expanding magnetic flux tube with a reconnecting current sheet trailing behind it, topologically analogous to coronal mass ejections. We predict two novel classes of electromagnetic precursor signals powered by the efficient dissipation of magnetic energy in these periodically forming trailing current sheets. First, particles accelerated in the sheets produce nonthermal gamma-ray signals peaking at $\sim16\,\mathrm{MeV}$, which escape minutes to seconds before merger while the sheets are still optically thin to pair production, with modest characteristic luminosities of $L_\mathrm{obs}\gtrsim 10^{42}\,\mathrm{erg/s}$, detectable only for nearby mergers. Second, merging plasmoids in the sheets could produce fast radio burst-like transients in the final seconds before merger, with characteristic luminosities $L_\mathrm{radio}\sim 10^{38-40}\,\mathrm{erg/s}$. These coherent radio precursors would be detectable by upcoming instruments, either in untargeted surveys by wide-field instruments such as CHORD, or through targeted follow-up of gravitational-wave early-warning alerts with instruments such as DSA or SKA-mid.

astro-ph.HE↗

arXiv2026

Synchrotron-cooled plasma distribution in the outer magnetosphere of a neutron star

The guiding center formalism is employed to analyze the motion of a charged relativistic particle in an inhomogeneous magnetic field, subject to magnetic mirroring and energy loss due to cooling. The governing equation for the evolution of the magnetic moment is derived. An example representing a neutron star (pulsar or magnetar) magnetosphere is presented to illustrate typical particle orbits. Notably, radiative losses are most pronounced near a trapped particle's turning point. Depending on the initial particle's pitch angle, energy loss can become catastrophic, resulting in the rapid migration of the particle into the loss cone and subsequent precipitation onto a neutron star. Conversely, particles with a larger pitch angle remain temporarily trapped and form a gradually decaying "cooled-loss-cone" or "funnel'' distribution, characterized by the maximum momentum space particle density being located at the edge of the loss cone. The size of the loss cone is energy-dependent and scales as $\alpha_{c} \propto \gamma^{3/10}$. Synchrotron losses are strongest in a well-localized region of the magnetosphere, about a few hundred to a thousand star radii under typical pulsar and magnetar conditions. This region is a plausible site for synchrotron radiation originating in the outer magnetosphere, and could also be responsible for non-polar coherent pulsar emission, as well as weak fast radio bursts.

astro-ph.HE↗

arXiv2025

Particle Acceleration in Magnetized Shear-Driven Turbulence

Shear flows, ubiquitous in space and astrophysical plasmas, can accelerate particles through turbulence excited by the Kelvin-Helmholtz instability. We present the first numerical study of particle acceleration in sustained, subsonic, non-relativistic, and magnetized turbulence driven purely by velocity shear, including full particle backreaction. Using two-dimensional MHD-PIC simulations with an initially uniform flow-aligned magnetic field and external stirring force, we demonstrate that, in this subsonic regime, sustained particle acceleration requires continuously driven turbulence, whereas freely decaying turbulence rapidly depletes its energy reservoirs and halts the acceleration. The acceleration mechanism operates through the systematic distortion of gyro-orbits by turbulent electric fields: acceleration phases extend the particle trajectory along the electric force, increasing the energy gain, while deceleration phases shorten the trajectory, reducing the energy loss. This asymmetry produces net energy gain despite stochastic fluctuations, with the mean energy change scaling quadratically with shear velocity, characteristic of second-order Fermi acceleration. Initially monoenergetic particles develop substantial non-thermal tails after the turbulence onset. The energization of particles that repeatedly cross shear layers follows geometric Brownian motion, yielding a log-normal distribution. High-energy particles exhibit pitch-angle anisotropy, becoming preferentially perpendicular to the flow-aligned magnetic field as their gyroradii exceed the turbulent layer width. These results establish shear-driven turbulence as a viable particle acceleration mechanism, providing a general model for particle energization in shear flows.

astro-ph.HE↗

arXiv2025

Transition to Petschek Reconnection in Subrelativistic Pair Plasmas: Implications for Particle Acceleration

While relativistic magnetic reconnection in pair plasmas has emerged in recent years as a candidate for the origin of radiation from extreme astrophysical environments, the corresponding subrelativistic pair plasma regime has remained less explored, leaving open the question of how relativistic physics affects reconnection. In this paper, we investigate the differences between these regimes by contrasting 2D particle-in-cell simulations of reconnection in pair plasmas with relativistic magnetization ($\sigma \gg 1$) and subrelativistic magnetization ($\sigma < 1$). By utilizing unprecedentedly large domain sizes and outflow boundary conditions, we demonstrate that lowering the magnetization results in a change in the reconnection geometry from a plasmoid chain to a Petschek geometry, where laminar exhausts bounded by slow-mode shocks emanate from a single diffusion region. We attribute this change to the reduced plasmoid production rate in the low-$\sigma$ case: when the secondary tearing rate is sufficiently low, plasmoids are too few in number to prevent the system from relaxing into a stable Petschek configuration. This geometric change also affects particle energization: we show that while high-$\sigma$ plasmoid chains generate power-law energy spectra, low-$\sigma$ Petschek exhausts merely heat incoming plasma and yield negligible nonthermal acceleration. These results have implications for predicting the global current sheet geometry and the resulting energy spectrum in a variety of systems.

physics.plasm-ph↗

arXiv2025

The role of three-dimensional effects on ion injection and acceleration in perpendicular shocks

Understanding the conditions that enable particle acceleration at non-relativistic collisionless shocks is essential to unveil the origin of cosmic rays. We employ 2D and 3D hybrid simulations (with kinetic ions and fluid electrons) to explore particle acceleration and magnetic field amplification in non-relativistic perpendicular shocks, focusing on the role of shock drift acceleration and its dependence on the shock Mach number. We perform an analysis of the ion injection process and demonstrate why efficient acceleration is only observed in 3D. In particular, we show that ion injection critically depends on the "porosity" of the magnetic turbulence in the downstream region near the shock, a property describing how easily the post-shock region allows particles to traverse it and return upstream without being trapped. This effect can only be properly captured in 3D. Additionally, we explore the impact of numerical resolution on ion energization, highlighting how resolving small-scale turbulence -- on scales below the thermal ion gyroradius -- is essential for accurately modeling particle injection. Overall, our results emphasize the necessity of high-resolution 3D simulations to capture the fundamental microphysics driving particle acceleration at perpendicular shocks.

astro-ph.HE↗

arXiv2025

Speed-dependent Threshold for Electron Injection into Diffusive Shock Acceleration

Finding the injection threshold for diffusive shock acceleration (DSA) of electrons in collisionless shocks has been a longstanding unsolved problem. Using first-principles kinetic simulations, we identify the conditions for electron injection into DSA and quantify the evolution of the nonthermal tail in self-generated electromagnetic turbulence. By analyzing electron trajectories and their momentum gain during shock-recrossing cycles, we demonstrate that electrons start participating in DSA when their speed is large enough to overrun the shock. We develop a minimal model showing that speed-dependent injection reproduces nonthermal electron spectra observed in kinetic simulations. Our findings establish a new criterion for electron DSA, which has broad implications for the nonthermal emission of shock-powered space/astrophysical systems.

astro-ph.HE↗

arXiv2024

Electron Acceleration at Quasi-parallel Non-relativistic Shocks: A 1D Kinetic Survey

We present a survey of 1D kinetic particle-in-cell simulations of quasi-parallel non-relativistic shocks to identify the environments favorable for electron acceleration. We explore an unprecedented range of shock speeds $v_{\rm sh}\approx 0.067-0.267\,c$, Alfv\'{e}n Mach numbers $\mathcal{M}_{\rm A} = 5-40$, sonic Mach numbers $\mathcal{M}_{\rm s} = 5-160$, as well as the proton-to-electron mass ratios $m_{\rm i}/m_{\rm e}=16-1836$. We find that high Alfv\'{e}n Mach number shocks can channel a large fraction of their kinetic energy into nonthermal particles, self-sustaining magnetic turbulence and acceleration to larger and larger energies. The fraction of injected particles is $\lesssim 0.5\%$ for electrons and $\approx 1\%$ for protons, and the corresponding energy efficiencies are $\lesssim 2\%$ and $\approx 10\%$, respectively. The extent of the nonthermal tail is sensitive to the Alfv\'{e}n Mach number; when $\mathcal{M}_{\rm A}\lesssim 10$, the nonthermal electron distribution exhibits minimal growth beyond the average momentum of the downstream thermal protons, independently of the proton-to-electron mass ratio. Acceleration is slow for shocks with low sonic Mach numbers, yet nonthermal electrons still achieve momenta exceeding the downstream thermal proton momentum when the shock Alfv\'{e}n Mach number is large enough. We provide simulation-based parametrizations of the transition from thermal to nonthermal distribution in the downstream (found at a momentum around $p_{\rm i,e}/m_{\rm i}v_{\rm sh} \approx 3\sqrt{m_{\rm i,e}/m_{\rm i}}$), as well as the ratio of nonthermal electron to proton number density. The results are applicable to many different environments and are important for modeling shock-powered nonthermal radiation.

astro-ph.HE↗

arXiv2024

SLAMS-Propelled Electron Acceleration at High-Mach Number Astrophysical Shocks

Short Large Amplitude Magnetic Structures (SLAMS) are frequently detected during spacecraft crossings over the Earth bow shock. We investigate the existence of such structures at astrophysical shocks, where they could result from the steepening of cosmic-ray (CR) driven waves. Using kinetic particle-in-cell simulations, we study the growth of SLAMS and the appearance of associated transient shocks in the upstream region of quasi-parallel, non-relativistic, high-Mach number collisionless shocks. We find that high-energy CRs significantly enhance the transverse magnetic field within SLAMS, producing highly inclined field lines. As SLAMS are advected towards the shock, these fields lines form an intermittent superluminal configuration which traps magnetized electrons at fast shocks. Due to their oscillatory nature, SLAMS are periodically separated by subluminal gaps with lower transverse magnetic field strength. In these regions, electrons diffuse and accelerate by bouncing between the shock and the approaching SLAMS region through a mechanism that we call quasi-periodic shock acceleration (QSA). We analytically derive the distribution of electrons accelerated via QSA, $f(p)\sim p^{[-4.7,-5.7]}$, which agrees well with the simulation spectra. We find that the electron power law remains steep until the end of our longest runs, providing a possible explanation for the steep electron spectra observed at least up to GeV energies in young and fast supernova remnants.

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 $\chi$ 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 $\chi=0^\circ$, 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 $\chi=90^\circ$, 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 ($\chi=60^\circ$) 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

Electron-Ion Temperature Ratio in Transrelativistic Unmagnetized Shock Waves

Weakly magnetized shock waves are paramount to a large diversity of environments, including supernova remnants, blazars, and binary-neutron-star mergers. Understanding the distribution of energy between electrons and ions within these astrophysical shock waves spanning a wide spectrum of velocities is a long-standing challenge. In this study, we present a unified model for the downstream electron temperature within unmagnetized shock waves. Encompassing velocities from Newtonian to relativistic, we probe regimes representative of the gradual deceleration of the forward shock in the late gamma-ray burst afterglow phase, such as GRB 170817A. In our model, heating results from an ambipolar electric field generated by the difference in inertia between electrons and ions, coupled with rapid electron scattering in the decelerating turbulence. Our findings demonstrate that the electron temperature consistently represents $10\%$ of the incoming ion kinetic energy in the shock front frame over the full range of shock velocities.

astro-ph.HE↗

arXiv2024

Dissecting the Crab Nebula with JWST: Pulsar wind, dusty filaments, and Ni/Fe abundance constraints on the explosion mechanism

We present JWST observations of the Crab Nebula, the iconic remnant of the historical SN 1054. The observations include NIRCam and MIRI imaging mosaics, plus MIRI/MRS IFU spectra that probe two select locations within the ejecta filaments. We derive a high-resolution map of dust emission and show that the grains are concentrated in the innermost, high-density filaments. These dense filaments coincide with multiple synchrotron bays around the periphery of the Crab's pulsar wind nebula (PWN). We measure synchrotron spectral index changes in small-scale features within the PWN's torus region, including the well-known knot and wisp structures. The index variations are consistent with Doppler boosting of emission from particles with a broken power-law distribution, providing the first direct evidence that the curvature in the particle injection spectrum is tied to the acceleration mechanism at the termination shock. We detect multiple nickel and iron lines in the ejecta filaments and use photoionization models to derive nickel-to-iron abundance ratios that are a factor of 3-8 higher than the solar ratio. We also find that the previously reported order-of-magnitude higher Ni/Fe values from optical data are consistent with the lower values from JWST when we reanalyze the optical emission using updated atomic data and account for local extinction from dust. We discuss the implications of our results for understanding the nature of the explosion that produced the Crab Nebula and conclude that the observational properties are most consistent with a low-mass iron-core-collapse supernova, even though an electron-capture explosion cannot be ruled out.

astro-ph.HE↗

arXiv2024

Electron heating in high Mach number collisionless shocks

The energy partition in high Mach number collisionless shock waves is central to a wide range of high-energy astrophysical environments. We present a new theoretical model for electron heating that accounts for the energy exchange between electrons and ions at the shock. The fundamental mechanism relies on the difference in inertia between electrons and ions, resulting in differential scattering of the particles off a decelerating magnetically-dominated microturbulence across the shock transition. We show that the self-consistent interplay between the resulting ambipolar-type electric field and diffusive transport of electrons leads to efficient heating in the magnetic field produced by the Weibel instability in the high-Mach number regime and is consistent with fully kinetic simulations.

astro-ph.HE↗

arXiv2024

Collisionless conduction in a high-beta plasma: a collision operator for whistler turbulence

The regulation of electron heat transport in high-beta, weakly collisional, magnetized plasma is investigated. A temperature gradient oriented along a mean magnetic field can induce a kinetic heat-flux-driven whistler instability (HWI), which back-reacts on the transport by scattering electrons and impeding their flow. Previous analytical and numerical studies have shown that the heat flux for the saturated HWI scales as the inverse of electron beta. These numerical studies, however, had limited scale separation and consequently large fluctuation amplitudes, which calls into question their relevance at astrophysical scales. To this end, we perform a series of particle-in-cell simulations of the HWI across a range of electron beta and temperature-gradient length scales under two different physical setups. The saturated heat flux in all of our simulations follows the expected inverse-electron-beta scaling, supporting the robustness of the result. We also use our simulation results to develop and implement several methods to construct an effective collision operator for whistler turbulence. The results point to an issue with the standard quasi-linear explanation of HWI saturation, which is analogous to the well-known 90-degree scattering problem in the cosmic ray community. Despite this limitation, the methods developed here can serve as a blueprint for future work seeking to characterize the effective collisionality caused by kinetic instabilities.

astro-ph.HE↗

arXiv2024

Long-term Evolution of Relativistic Unmagnetized Collisionless Shocks

We study a relativistic collisionless electron-positron shock propagating into an unmagnetized ambient medium using 2D particle-in-cell simulations of unprecedented duration and size. The shock generates intermittent magnetic structures of increasingly larger size as the simulation progresses. Toward the end of our simulation, at around 26,000 plasma times, the magnetic coherence scale approaches $\lambda\sim 100$ plasma skin depths, both ahead and behind the shock front. We anticipate a continued growth of $\lambda$ beyond the time span of our simulation, as long as the shock accelerates particles to increasingly higher energies. The post-shock field is concentrated in localized patches, which maintain a local magnetic energy fraction $\varepsilon_B\sim 0.1$. Particles randomly sampling the downstream fields spend most of their time in low field regions ($\varepsilon_B\ll 0.1$), but emit a large fraction of the synchrotron power in the localized patches with strong fields ($\varepsilon_B\sim 0.1$). Our results have important implications for models of gamma-ray burst afterglows.

astro-ph.HE↗

arXiv2023

Return Currents in Collisionless Shocks

Collisionless shocks tend to send charged particles into the upstream, driving electric currents through the plasma. Using kinetic particle-in-cell simulations, we investigate how the background thermal plasma neutralizes such currents in the upstream of quasi-parallel non-relativistic electron-proton shocks. We observe distinct processes in different regions: the far upstream, the shock precursor, and the shock foot. In the far upstream, the current is carried by nonthermal protons, which drive electrostatic modes and produce supra-thermal electrons that move towards upstream infinity. Closer to the shock (in the precursor), both the current density and the momentum flux of the beam increase, which leads to electromagnetic streaming instabilities that contribute to the thermalization of supra-thermal electrons. At the shock foot, these electrons are exposed to shock-reflected protons, resulting in a two-stream type instability. We analyze these processes and the resulting heating through particle tracking and controlled simulations. In particular, we show that the instability at the shock foot can make the effective thermal speed of electrons comparable to the drift speed of the reflected protons. These findings are important for understanding both the magnetic field amplification and the processes that may lead to the injection of supra-thermal electrons into diffusive shock acceleration.

astro-ph.HE↗

arXiv2023

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 $\tilde{\mu}_0$ of flux tube field lines determines the growth rate ($\propto \tilde{\mu}_0^3$) and minimum wavelength of the kink instability ($\propto \tilde{\mu}_0^{-1}$). We use these scalings to determine a minimum flux tube length for the growth of the kink instability for any given $\tilde{\mu}_0$. 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↗