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 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 in periodically spaced thin sheets, and electromagnetic waves propagate between them as in a waveguide. We show the following. (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 nearvacuum regions.
Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher.
Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?
Some links on this page may take you to nonfederal websites. Their policies may differ from this site.

Abstract 
Abstract Spider pulsars are compact binary systems composed of a millisecond pulsar and a lowmass companion. The relativistic magnetically dominated pulsar wind impacts onto the companion, ablating it and slowly consuming its atmosphere. The interaction forms an intrabinary shock, a proposed site of particle acceleration. We perform global fully kinetic particleincell simulations of the intrabinary shock, assuming that the pulsar wind consists of planeparallel stripes of alternating polarity and that the shock wraps around the companion. We find that particles are efficiently accelerated via shockdriven reconnection. We extract firstprinciples synchrotron spectra and light curves, which are in good agreement with Xray observations: (1) the synchrotron spectrum is nearly flat,
F _{ν}∝ const; (2) when the pulsar spin axis is nearly aligned with the orbital angular momentum, the light curve displays two peaks, just before and after the pulsar eclipse (pulsar superior conjunction), separated in phase by ∼0.8 rad; (3) the peak flux exceeds the one at the inferior conjunction by a factor of 10. 
Abstract We perform particleincell simulations to elucidate the microphysics of relativistic weakly magnetized shocks loaded with electronpositron pairs. Various external magnetizations
σ ≲ 10^{−4}and pairloading factorsZ _{±}≲ 10 are studied, whereZ _{±}is the number of loaded electrons and positrons per ion. We find the following: (1) The shock becomes mediated by the ion Larmor gyration in the mean field whenσ exceeds a critical valueσ _{L}that decreases withZ _{±}. Atσ ≲σ _{L}the shock is mediated by particle scattering in the selfgenerated microturbulent fields, the strength and scale of which decrease withZ _{±}, leading to lowerσ _{L}. (2) The energy fraction carried by the postshock pairs is robustly in the range between 20% and 50% of the upstream ion energy. The mean energy per postshock electron scales as . (3) Pair loading suppresses nonthermal ion acceleration at magnetizations as low as ${\overline{E}}_{\mathrm{e}}\propto {\left({Z}_{\pm}+1\right)}^{1}$σ ≈ 5 × 10^{−6}. The ions then become essentially thermal with mean energy , while electrons form a nonthermal tail, extending from ${\overline{E}}_{\mathrm{i}}$ to $E\sim {\left({Z}_{\pm}+1\right)}^{1}{\overline{E}}_{\mathrm{i}}$ . When ${\overline{E}}_{\mathrm{i}}$σ = 0, particle acceleration is enhanced by the formation of intense magnetic cavities that populate the precursor during the late stages of shock evolution. Here,more » 
Free, publiclyaccessible full text available April 1, 2023

Free, publiclyaccessible full text available April 1, 2023

ABSTRACT Magnetars are the most promising progenitors of fast radio bursts (FRBs). Strong radio waves propagating through the magnetar wind are subject to nonlinear 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 a0 ≲ 1, and discuss implications for FRBs. As an effect of the instability, the FRBradiation 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 FRBscattering timescale depends on the properties of the magnetar wind. In a cold wind, the typical scattering timescale is τsc ∼ $\mu$s–ms at the frequency $\nu \sim 1\, {\rm GHz}$. The scattering timescale increases at low frequencies, with the scaling τsc ∝ ν−2. The frequencydependent 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 $\tau _{\rm sc}\sim \, {\rm ns}$. Then scattering produces a frequency modulation of the observed intensitymore »

Abstract Magnetic reconnection is invoked as one of the primary mechanisms to produce energetic particles. We employ largescale 3D particleincell simulations of reconnection in magnetically dominated ( σ = 10) pair plasmas to study the energization physics of highenergy particles. We identify an acceleration mechanism that only operates in 3D. For weak guide fields, 3D plasmoids/flux ropes extend along the z direction of the electric current for a length comparable to their crosssectional radius. Unlike in 2D simulations, where particles are buried in plasmoids, in 3D we find that a fraction of particles with γ ≳ 3 σ can escape from plasmoids by moving along z , and so they can experience the largescale fields in the upstream region. These “free” particles preferentially move in z along Speiserlike orbits sampling both sides of the layer and are accelerated linearly in time—their Lorentz factor scales as γ ∝ t , in contrast to γ ∝ t in 2D. The energy gain rate approaches ∼ eE rec c , where E rec ≃ 0.1 B 0 is the reconnection electric field and B 0 the upstream magnetic field. The spectrum of free particles is hard, dN free / d γ ∝ γmore »

Abstract We introduce a toy model for the time–frequency structure of fast radio bursts, in which the observed emission is produced as a narrowly peaked intrinsic spectral energy distribution sweeps down in frequency across the instrumental bandpass as a power law in time. Though originally motivated by emission models that invoke a relativistic shock, the model could in principle apply to a wider range of emission scenarios. We quantify the burst’s detectability using the frequency bandwidth over which most of its signaltonoise ratio is accumulated. We demonstrate that, by varying just a single parameter of the toy model—the powerlaw index
β of the frequency drift rate—one can transform a long (and hence preferentially timeresolved) burst with a narrow timeintegrated spectrum into a shorter burst with a broad powerlaw timeintegrated spectrum. We suggest that sourcetosource diversity in the value ofβ could generate the dichotomy between burst duration and frequencybandwidth recently found by CHIME/FRB. In shock models, the value ofβ is related to the radial density profile of the external medium, which, in light of the preferentially longer duration of bursts from repeating sources, may point to diversity in the external environments surrounding repeating versus oneoff FRB sources. 
Comptonization by reconnection plasmoids in black hole coronae I: Magnetically dominated pair plasmaABSTRACT We perform 2D particleincell simulations of reconnection in magnetically dominated electron–positron plasmas subject to strong Compton cooling. We vary the magnetization σ ≫ 1, defined as the ratio of magnetic tension to plasma inertia, and the strength of cooling losses. Magnetic reconnection under such conditions can operate in magnetically dominated coronae around accreting black holes, which produce hard Xrays through Comptonization of seed soft photons. We find that the particle energy spectrum is dominated by a peak at mildly relativistic energies, which results from bulk motions of cooled plasmoids. The peak has a quasiMaxwellian shape with an effective temperature of ∼100 keV, which depends only weakly on the flow magnetization and the strength of radiative cooling. The mean bulk energy of the reconnected plasma is roughly independent of σ, whereas the variance is larger for higher magnetizations. The spectra also display a highenergy tail, which receives ∼25 per cent of the dissipated reconnection power for σ = 10 and ∼40 per cent for σ = 40. We complement our particleincell studies with a Monte Carlo simulation of the transfer of seed soft photons through the reconnection layer, and find the escaping Xray spectrum. The simulation demonstrates that Comptonization is dominated by the bulk motions in themore »