arXiv subjects

Yuri Lyubarsky

Publications and source records attributed to Yuri Lyubarsky.

At least 19 recordsLinked to original sources

arXiv2025

Cavitation instability in unmagnetized relativistic pair shocks

We investigate the formation of plasma cavities in unmagnetized relativistic pair shocks and demonstrate that these cavities emerge naturally as a nonlinear consequence of asymmetric Weibel instability. We provide an intuitive physical picture and a minimal fluid model that captures the essential features of this cavitation instability and compare them with PIC results. This mechanism may provide the missing link between kinetic Weibel turbulence and macroscopic magnetic fields in astrophysical shocks.

astro-ph.HE↗

arXiv2025

Filamentation of the electromagnetic precursor in relativistic quasi-perpendicular electron-positron shocks

We present a scenario that could explain non-thermal particle acceleration in relativistic quasi-perpendicular electron-positron shocks, such as the termination shock of pulsar wind nebulae. The shock produces a strong electromagnetic precursor that propagates into the upstream plasma, which is initially threaded by a uniform background magnetic field. We show that the filamentation instability breaks the precursor into radiation filaments parallel to the shock normal. The transverse scale of the filaments is of the order of a few plasma skin depths. In the shock frame, the bulk Lorentz factor of the upstream plasma is significantly reduced inside the radiation filaments. Then, the instability produces a relativistic shear flow with strong velocity gradients on kinetic scales. The velocity gradients distort the background magnetic field lines, and generate a magnetic field component parallel to the shock normal that reverses across each radiation filament, a configuration that could trigger magnetic reconnection in the upstream plasma. These effects may accelerate particles before the plasma enters the shock.

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arXiv2024

Collisionless tearing instability in relativistic non-thermal pair plasma and its application to MHD turbulence

Collisionless tearing instability with a power-law distribution function in a relativistic pair plasma with a guide field is studied. When the current sheet is supported by plasma pressure, the tearing mode is suppressed as the particle spectrum hardens. In the force-free limit, the instability growth rate becomes independent of the particle spectrum. We apply these results to relativistic MHD turbulence, where magnetic energy greatly exceeds plasma rest energy, and derive an expression for the transverse size of turbulent eddies unstable to tearing mode. We also establish the critical plasma magnetization parameter above which charge starvation prevents the tearing instability. These results might be useful for developing more accurate models of particle acceleration in relativistic astrophysical sources.

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arXiv2024

Collisionless tearing instability in a relativistic pair plasma with a power-law distribution function

We study the tearing instability of a current sheet in a relativistic pair plasma with a power law distribution function. We first estimate the growth rate analytically and then confirm the analytical results by solving numerically the dispersion equation, taking into account all exact particle trajectories within the reconnecting layer. We found that the instability is suppressed when the particle spectrum becomes harder.

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arXiv2023

The precursor structure in relativistic shocks

We present a common unifying macroscopic framework for precursors in relativistic shock waves. These precursors transfer energy and momentum from the hot downstream to the cold upstream, modifying the shock structure. Derishev & Piran (2016} have shown that in a steady state, there is a maximal fraction of the downstream energy flux that the precursor can carry. We show that at this critical value, the shock disappears, and the flow passes through a sonic point. This behavior resembles the classical Newtonian Rayleigh flow problem. At the critical value, the transition is unstable as perturbations in the upstream accumulate at the sonic point. Thus, if such a point is reached, the shock structure is drastically modified, and the flow becomes turbulent.

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arXiv2023

On the escape of low-frequency waves from magnetospheres of neutron stars

We study the nonlinear decay of the fast magnetosonic into the Alfvén waves in relativistic force-free magnetohydrodynamics. The work has been motivated by models of pulsar radio emission and fast radio bursts (FRBs), in which the emission is generated in neutron star magnetospheres at conditions when not only the Larmor but also the plasma frequencies significantly exceed the radiation frequency. The decay process places limits on the source luminosity in these models. We estimated the decay rate and showed that the phase volume of Alfvén waves available for the decay of an fms wave is infinite. Therefore the energy of fms waves could be completely transferred to the small-scale Alfvén waves not via a cascade, as in the Kolmogorov turbulence, but directly. Our results explain the anomalously low radio efficiency of the Crab pulsar and show that FRBs could not be produced well within magnetar magnetospheres.

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arXiv2023

The collimation of relativistic jets in post-neutron star binary merger simulations

The gravitational waves from the binary neutron star merger GW170817 were accompanied by a multi-wavelength electromagnetic counterpart, which confirms the association of the merger with a short gamma-ray burst (sGRB). The afterglow observations implied that the event was accompanied by a narrow, $\sim 5~$deg, and powerful, $\sim 10^{50}$ erg, jet. We study the propagation of a Poynting flux-dominated jet within the merger ejecta (kinematic, neutrino-driven and MRI turbulence-driven) of a neutrino-radiation-GR-MHD simulation of two coalescing neutron stars. We find that the presence of a post-merger low-density/low-pressure polar cavity, that arose due to angular momentum conservation, is crucial to let the jet break out. At the same time the ejecta collimates the jet to a narrow opening angle. The collimated jet has a narrow opening angle of $\sim 4$-$7$ deg and an energy of $10^{49}$-$10^{50}~$erg, in line with the observations of GW170817 and other sGRBs.

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arXiv2023

Saturation of the filamentation instability and dispersion measure of Fast Radio Bursts

Nonlinear effects are crucial for the propagation of Fast Radio Bursts (FRBs) near the source. We study the filamentation of FRBs in the relativistic winds of magnetars, which are commonly invoked as the most natural FRB progenitors. As a result of filamentation, the particle number density and the radiation intensity develop strong gradients along the direction of the wind magnetic field. A steady state is reached when the plasma pressure balances the ponderomotive force. In such a steady state, particles are confined into periodically spaced thin sheets, and electromagnetic waves propagate between them as in a waveguide. We show that: (i) The dispersion relation resembles that in the initial homogeneous plasma, but the effective plasma frequency is determined by the separation of the sheets, not directly by the mean particle density. (ii) The contribution of relativistic magnetar winds to the dispersion measure of FRBs could be several orders of magnitude larger than previously thought. The dispersion measure of the wind depends on the properties of individual bursts (e.g. the luminosity), and therefore can change significantly among different bursts from repeating FRBs. (iii) Induced Compton scattering is suppressed because most of the radiation propagates in near vacuum regions.

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arXiv2022

Radiatively driven evaporation from magnetar's surface

The luminosity of the Soft Gamma Repeater (SGR) flares significantly exceeds the Eddington luminosity. This is because they emit mainly in the E-mode, for which the radiative cross-sections are strongly suppressed. The energy is released in the magnetosphere forming a magnetically trapped pair fireball, and the surface of the star is illuminated by the powerful radiation from the fireball. We study the ablation of the matter from the surface by this radiation. The E-mode photons are scattered within the surface layer, partly being converted into O-photons, whose scattering cross-section is of the order of the Thomson cross-section. The high radiation pressure of the O-mode radiation expels the plasma upwards. The uplifted matter forms a thick baryon sheath around the fireball. If an illuminated fraction of the star's surface includes the polar cap, a heavy, mildly relativistic baryonic wind is formed.

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arXiv2022

Filamentation of Fast Radio Bursts in magnetar winds

Magnetars are the most promising progenitors of Fast Radio Bursts (FRBs). Strong radio waves propagating through the magnetar wind are subject to non-linear effects, including modulation/filamentation instabilities. We derive the dispersion relation for modulations of strong waves propagating in magnetically-dominated pair plasmas focusing on dimensionless strength parameters $a_0\lesssim 1$, and discuss implications for FRBs. As an effect of the instability, the FRB radiation intensity develops sheets perpendicular to the direction of the wind magnetic field. When the FRB front expands outside the radius where the instability ends, the radiation sheets are scattered due to diffraction. The FRB scattering timescale depends on the properties of the magnetar wind. In a cold wind, the typical scattering timescale is $τ_{\rm sc}\sim{\rm\; μs-ms}$ at the frequency $ν\sim 1{\rm\; GHz}$. The scattering timescale increases at low frequencies, with the scaling $τ_{\rm sc}\proptoν^{-2}$. The frequency-dependent broadening of the brightest pulse of FRB 181112 is consistent with this scaling. From the scattering timescale of the pulse, one can estimate that the wind Lorentz factor is larger than a few tens. In a warm wind, the scattering timescale can approach $τ_{\rm sc}\sim{\rm\; ns}$. Then scattering produces a frequency modulation of the observed intensity with a large bandwidth, $Δν\sim 1/τ_{\rm sc}\gtrsim 100{\rm\; MHz}$. Broadband frequency modulations observed in FRBs could be due to scattering in a warm magnetar wind.

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arXiv2021

Nonlinear electromagnetic-wave interactions in pair plasma: (I) Non-relativistic regime

This paper is the first in a series devoted to the numerical study of nonlinear interactions of electromagnetic waves with plasma. We start with non-magnetized pair plasmas, where the primary processes are induced (Compton) scattering and the filamentation instability. In this paper, we consider waves in which electron oscillations are non-relativistic. Here, the numerical results can be compared to analytical theory, facilitating the development of appropriate numerical tools and framework. We distill the analytic theory, reconciling plasma and radiative transfer pictures of induced scattering and developing in detail the kinetic theory of modulation/filamentation instability. We carry out homogeneous numerical simulations using the particle-in-cell codes EPOCH and Tristan-MP, for both monochromatic waves and wave packets. We show that simulations of both processes are consistent with theoretical predictions, setting the stage for analyzing the highly nonlinear regime.

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arXiv2021

Emission Mechanisms of Fast Radio Bursts

Fast radio bursts (FRBs) are recently discovered mysterious single pulses of radio emission, mostly coming from cosmological distances ($\sim 1$ Gpc). Their short duration, $\sim 1$ ms, and large luminosity evidence coherent emission. I review the basic physics of coherent emission mechanisms proposed for FRBs. In particular, I discuss the curvature emission of bunches, the synchrotron maser, and the emission of radio waves by variable currents in the course of magnetic reconnection. Special attention is paid to magnetar flares as the most promising sources of FRBs. Non-linear effects are outlined that could place bounds on the power of the outgoing radiation.

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arXiv2021

The role of resonant plasma instabilities in the evolution of blazar induced pair beams

The fate of relativistic pair beams produced in the intergalactic medium by very high energy emission from blazars remains controversial in the literature. The possible role of resonance beam plasma instability has been studied both analytically and numerically but no consensus has been reached. In this paper, we thoroughly analyze the development of this type of instability. This analysis takes into account that a highly relativistic beam loses energy only due to interactions with the plasma waves propagating within the opening angle of the beam (we call them parallel waves), whereas excitation of oblique waves results merely in an angular spreading of the beam, which reduces the instability growth rate. For parallel waves, the growth rate is a few times larger than for oblique ones, so they grow faster than oblique waves and drain energy from the beam before it expands. However, the specific property of extragalactic beams is that they are extraordinarily narrow; the opening angle is only $Δθ\sim 10^{-6}-10^{-5}$. In this case, the width of the resonance for parallel waves, $\proptoΔθ^2$, is too small for them to grow in realistic conditions. We perform both analytical estimates and numerical simulations in the quasilinear regime. These show that for extragalactic beams, the growth of the waves is incapable of taking a significant portion of the beam's energy. This type of instability could at best lead to an expansion of the beam by some factor but the beam's energy remains nearly intact.

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arXiv2020

Self-modulation of Fast Radio Bursts

Fast Radio Bursts (FRBs) are extreme astrophysical phenomena entering the realm of non-linear optics, a field developed in laser physics. A classical non-linear effect is self-modulation. We examine the propagation of FRBs through the circumburst environment using the idealised setup of a monochromatic linearly-polarised GHz wave propagating through a uniform plasma slab of density $N$ at distance $R$ from the source. We find that self-modulation occurs if the slab is located within a critical radius $R_{\rm crit}\sim 10^{17}(N/10^2{\rm\; cm}^{-3})(L/10^{42}{\rm\; erg\; s}^{-1}){\rm\; cm}$, where $L$ is the isotropic equivalent of the FRB luminosity. Self-modulation breaks the burst into pancakes transverse to the radial direction. When $R\lesssim R_{\rm crit}$, the transverse size of the pancakes is smaller than the Fresnel scale. The pancakes are strongly diffracted as the burst exits the slab, and interference between the pancakes produces a frequency modulation of the observed intensity with a sub-GHz bandwidth. When $R\sim R_{\rm crit}$, the transverse size of the pancakes becomes comparable with the Fresnel scale, and the effect of diffraction is weaker. The observed intensity is modulated on a timescale of ten microseconds, which corresponds to the radial width of the pancakes. Our results suggest that self-modulation may cause the temporal and frequency structure observed in FRBs.

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arXiv2020

Spectral Modification of Magnetar Flares by Resonant Cyclotron Scattering

Spectral modification of energetic magnetar flares by resonant cyclotron scattering (RCS) is considered. During energetic flares, photons emitted from the magnetically-trapped fireball near the stellar surface should resonantly interact with magnetospheric electrons or positrons. We show by a simple thought experiment that such scattering particles are expected to move at mildly relativistic speeds along closed magnetic field lines, which would slightly shift the incident photon energy due to the Doppler effect. We develop a toy model for the spectral modification by a single RCS that incorporates both a realistic seed photon spectrum from the trapped fireball and the velocity field of particles, which is unique to the flaring magnetosphere. We show that our spectral model can be effectively characterized by a single parameter; the effective temperature of the fireball, which enables us to fit observed spectra with low computational cost. We demonstrate that our single scattering model is in remarkable agreement with Swift/BAT data of intermediate flares from SGR 1900+14, corresponding to effective fireball temperatures of $T_{\rm eff}=6$-$7$ keV, whereas BeppoSAX/GRBM data of giant flares from the same source may need more elaborate models including the effect of multiple scatterings. Nevertheless, since there is no standard physically-motivated model for magnetar flare spectra, our model could be a useful tool to study magnetar bursts, shedding light on the hidden properties of the flaring magnetosphere.

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arXiv2020

Fast radio bursts from reconnection in magnetar magnetosphere

The nearly 100% linear polarization has been reported for a few fast radio bursts. This finding places severe limits on the emission mechanism. I argue that the totally polarized radiation could be generated in the course of relativistic magnetic reconnection in the outer magnetosphere of the magnetar. At the onset of the magnetar flare, a large scale magnetic perturbation forms a magnetic pulse, which propagates from the flare cite outwards. The pulse strongly compresses magnetospheric plasma and pushes it away. The high-frequency MHD waves are generated when the magnetic pulse reaches the current sheet separating, just beyond the light cylinder, the oppositely directed magnetic fields. Coalescence of magnetic islands in the reconnecting current sheet produces magnetosonic waves, which propagate away on the top of the magnetic pulse and escape in the far zone of the wind as radio waves polarized along the rotational axis of the magnetar. I estimate parameters of the outgoing radiation and show that they are compatible with the observed properties of FRBs.

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arXiv2018

Radio emission from the Crab and Crab-like pulsars

The pulsar radio emission is commonly associated with the plasma outflow in the open field line tube; then a pencil beam is emitted along the pulsar magnetic axis. Observations suggest that there is an additional radio emission mechanism specific for pulsars with high magnetic field at the light cylinder. These pulsars are known to be strong sources of non-thermal high energy radiation, which could be attributed to reconnection in the current sheet separating, just beyond the light cylinder, the oppositely directed magnetic fields. Pulsars with the highest magnetic field at the light cylinder (>100 kG) exhibit also radio pulses in phase with the high energy pulses. Moreover, giant radio pulses are observed in these pulsars. I argue that the reconnection process that produces high energy emission could also be responsible for the radio emission. Namely, coalescence of magnetic islands in the sheet produces magnetic perturbations that propagate away in the form of electro-magnetic nano-shots. I estimate the parameters of this emission and show that they are compatible with observations.

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arXiv2017

Interaction of the electro-magnetic precursor from a relativistic shock with the upstream flow. I. Synchrotron absorption of strong electromagnetic waves

This paper is the first in the series of papers aiming to study interaction of the electro-magnetic precursor waves generated at the front of a relativistic shock with the upstream flow. It is motivated by a simple consideration showing that the absorption of such an electro-magnetic precursor could yield an efficient transformation of the kinetic energy of the upstream flow to the energy of accelerated particles. Taking into account that the precursor is a strong wave, in which electrons oscillate with relativistic velocities, the standard plasma-radiation interaction processes should be reconsidered. In this paper, I calculate the synchrotron absorption of strong electro-magnetic waves.

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