arXiv subjects

Lorenzo Sironi

Publications and source records attributed to Lorenzo Sironi.

At least 73 records · Page 4Linked to original sources

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.

astro-ph.HE↗

arXiv2022

Comptonization by Reconnection Plasmoids in Black Hole Coronae II: Electron-Ion Plasma

We perform two-dimensional particle-in-cell simulations of magnetic reconnection in electron-ion plasmas subject to strong Compton cooling and calculate the X-ray spectra produced by this process. The simulations are performed for trans-relativistic reconnection with magnetization $1\leq σ\leq 3$ (defined as the ratio of magnetic tension to plasma rest-mass density), which is expected in the coronae of accretion disks around black holes. We find that magnetic dissipation proceeds with inefficient energy exchange between the heated ions and the Compton-cooled electrons. As a result, most electrons are kept at a low temperature in Compton equilibrium with radiation, and so thermal Comptonization cannot reach photon energies $\sim 100$ keV observed from accreting black holes. Nevertheless, magnetic reconnection efficiently generates $\sim 100$ keV photons because of mildly relativistic bulk motions of the plasmoid chain formed in the reconnection layer. Comptonization by the plasmoid motions dominates the radiative output and controls the peak of the radiation spectrum $E_{\rm pk}$. We find $E_{\rm pk}\sim 40$ keV for $σ=1$ and $E_{\rm pk}\sim100$ keV for $σ=3$. In addition to the X-ray peak around 100 keV, the simulations show a non-thermal MeV tail emitted by a non-thermal electron population generated near X-points of the reconnection layer. The results are consistent with the typical hard state of accreting black holes. In particular, we find that the spectrum of Cygnus~X-1 is well explained by electron-ion reconnection with $σ\sim 3$.

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arXiv2022

Ion and Electron Acceleration in Fully Kinetic Plasma Turbulence

Turbulence is often invoked to explain the origin of nonthermal particles in space and astrophysical plasmas. By means of 3D fully kinetic particle-in-cell simulations, we demonstrate that turbulence in low-$β$ plasmas ($β$ is the ratio of plasma pressure to magnetic pressure) accelerates ions and electrons into a nonthermal energy distribution with a power-law energy range. The ion spectrum is harder than the electron one, and both distributions get steeper for higher $β$. We show that the energization of electrons is accompanied by a significant energy-dependent pitch-angle anisotropy, with most electrons moving parallel to the local magnetic field, while ions stay roughly isotropic. We demonstrate that particle injection from the thermal pool occurs in regions of high current density. Parallel electric fields associated with magnetic reconnection are responsible for the initial energy gain of electrons, whereas perpendicular electric fields control the overall energization of ions. Our findings have important implications for the origin of nonthermal particles in space and astrophysical plasmas.

astro-ph.HE↗

arXiv2022

A Heating Mechanism via Magnetic Pumping in the Intracluster Medium

Turbulence driven by AGN activity, cluster mergers and galaxy motion constitutes an attractive energy source for heating the intracluster medium (ICM). How this energy dissipates into the ICM plasma remains unclear, given its low collisionality and high magnetization (precluding viscous heating by Coulomb processes). Kunz et al. 2011 proposed a viable heating mechanism based on the anisotropy of the plasma pressure (gyroviscous heating) under ICM conditions. The present paper builds upon that work and shows that particles can be gyroviscously heated by large-scale turbulent fluctuations via magnetic pumping. We study how the anisotropy evolves under a range of forcing frequencies, what waves and instabilities are generated and demonstrate that the particle distribution function acquires a high energy tail. For this, we perform particle-in-cell simulations where we periodically vary the mean magnetic field $\textbf{B}(t)$. When $\textbf{B}(t)$ grows (dwindles), a pressure anisotropy $P_{\perp}>P_{\parallel}$ ($P_{\perp}< P_{\parallel}$) builds up ($P_{\perp}$ and $P_{\parallel}$ are, respectively, the pressures perpendicular and parallel to $\textbf{B}(t)$). These pressure anisotropies excite mirror ($P_{\perp}>P_{\parallel}$) and oblique firehose ($P_{\parallel}>P_{\perp}$) instabilities, which trap and scatter the particles, limiting the anisotropy and providing a channel to heat the plasma. The efficiency of this mechanism depends on the frequency of the large-scale turbulent fluctuations and the efficiency of the scattering the instabilities provide in their nonlinear stage. We provide a simplified analytical heating model that captures the phenomenology involved. Our results show that this process can be relevant in dissipating and distributing turbulent energy at kinetic scales in the ICM.

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arXiv2022

Non-resonant particle acceleration in strong turbulence: comparison to kinetic and MHD simulations

Collisionless, magnetized turbulence offers a promising framework for the generation of non-thermal high-energy particles in various astrophysical sites. Yet, the detailed mechanism that governs particle acceleration has remained subject to debate. By means of 2D and 3D PIC, as well as 3D (incompressible) magnetohydrodynamic (MHD) simulations, we test here a recent model of non-resonant particle acceleration in strongly magnetized turbulence~\cite{2021PhRvD.104f3020L}, which ascribes the energization of particles to their continuous interaction with the random velocity flow of the turbulence, in the spirit of the original Fermi model. To do so, we compare, for a large number of particles that were tracked in the simulations, the predicted and the observed histories of particles momenta. The predicted history is that derived from the model, after extracting from the simulations, at each point along the particle trajectory, the three force terms that control acceleration: the acceleration of the field line velocity projected along the field line direction, its shear projected along the same direction, and its transverse compressive part. Overall, we find a clear correlation between the model predictions and the numerical experiments, indicating that this non-resonant model can successfully account for the bulk of particle energization through Fermi-type processes in strongly magnetized turbulence. We also observe that the parallel shear contribution tends to dominate the physics of energization in the PIC simulations, while in the MHD incompressible simulation, both the parallel shear and the transverse compressive term provide about equal contributions.

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arXiv2022

Relativistic Asymmetric Reconnection

We derive basic scaling equations for relativistic magnetic reconnection in the general case of asymmetric inflow conditions and obtain predictions for the outflow Lorentz factor and the reconnection rate. Kinetic Particle-in-Cell simulations show that the outflow speeds as well as the nonthermal spectral index are constrained by the inflowing plasma with the weaker magnetic energy per particle, in agreement with the scaling predictions. These results are significant for understanding non-thermal emission from reconnection in magnetically-dominated, astrophysical systems, many of which may be asymmetric in nature. The results provide a quantitative approach for including asymmetry on reconnection in the relativistic regime.

physics.plasm-ph↗

arXiv2022

Non-ideal fields solve the injection problem in relativistic reconnection

Magnetic reconnection in relativistic plasmas is well established as a fast and efficient particle accelerator, capable of explaining the most dramatic astrophysical flares. With particle-in-cell simulations, we demonstrate the importance of non-ideal fields for the early stages ("injection") of particle acceleration. Most of the particles ending up with high energies (near or above the mean magnetic energy per particle) must have passed through non-ideal regions where the assumptions of ideal magnetohydrodynamics are broken (i.e., regions with $E>B$ or nonzero $E_\parallel={\bf E}\cdot {\bf B}/B$), whereas particles that do not experience non-ideal fields end up with Lorentz factors of order unity. Thus, injection by non-ideal fields is a necessary prerequisite for further acceleration. Our results have important implications for the origin of nonthermal particles in high-energy astrophysical sources.

astro-ph.HE↗

arXiv2022

Global Kinetic Modeling of the Intrabinary Shock in Spider Pulsars

Spider pulsars are compact binary systems composed of a millisecond pulsar and a low-mass 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 particle-in-cell simulations of the intrabinary shock, assuming that the pulsar wind consists of plane-parallel stripes of alternating polarity and that the shock wraps around the companion. We find that particles are efficiently accelerated via shock-driven reconnection. We extract first-principles synchrotron spectra and lightcurves which are in good agreement with X-ray observations: (1) the synchrotron spectrum is nearly flat, $F_ν\propto {\rm const}$; (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 $\sim 0.8\, {\rm rad}$; (3) the peak flux exceeds the one at inferior conjunction by a factor of ten. We demonstrate that the double-peaked signature in the lightcurve is due to Doppler boosting in the post-shock flow.

astro-ph.HE↗

arXiv2022

Microphysics of Relativistic Collisionless Electron-ion-positron Shocks

We perform particle-in-cell simulations to elucidate the microphysics of relativistic weakly magnetized shocks loaded with electron-positron pairs. Various external magnetizations $σ\lesssim 10^{-4}$ and pair-loading factors $Z_\pm \lesssim 10$ are studied, where $Z_\pm$ 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 when $σ$ exceeds a critical value $σ_{\rm L}$ that decreases with $Z_\pm$. At $σ\lesssimσ_{\rm L}$ the shock is mediated by particle scattering in the self-generated microturbulent fields, the strength and scale of which decrease with $Z_\pm$, leading to lower $σ_{\rm L}$. (2) The energy fraction carried by the post-shock pairs is robustly in the range between 20% and 50% of the upstream ion energy. The mean energy per post-shock electron scales as $\overline{E}_{\rm e}\propto (Z_\pm+1)^{-1}$. (3) Pair loading suppresses nonthermal ion acceleration at magnetizations as low as $σ\approx 5\times 10^{-6}$. The ions then become essentially thermal with mean energy $\overline{E}_{\rm i}$, while electrons form a nonthermal tail, extending from $E\sim (Z_\pm + 1)^{-1}\overline{E}_{\rm i}$ to $\overline{E}_{\rm i}$. When $σ= 0$, particle acceleration is enhanced by the formation of intense magnetic cavities that populate the precursor during the late stages of shock evolution. Here, the maximum energy of the nonthermal ions and electrons keeps growing over the duration of the simulation. Alongside the simulations, we develop theoretical estimates consistent with the numerical results. Our findings have important implications for models of early gamma-ray burst afterglows.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2021

Stochastic Electron Acceleration by Temperature Anisotropy Instabilities Under Solar Flare Plasma Conditions

Using 2D particle-in-cell (PIC) plasma simulations we study electron acceleration by temperature anisotropy instabilities, assuming conditions typical of above-the-loop-top (ALT) sources in solar flares. We focus on the long-term effect of $T_{e,\perp} > T_{e,\parallel}$ instabilities by driving the anisotropy growth during the entire simulation time, through imposing a shearing or a compressing plasma velocity ($T_{e,\perp}$ and $T_{e,\parallel}$ are the temperatures perpendicular and parallel to the magnetic field). This magnetic growth makes $T_{e,\perp}/T_{e,\parallel}$ grow due to electron magnetic moment conservation, and amplifies the ratio $ω_{ce}/ω_{pe}$ from $\sim 0.53$ to $\sim 2$ ($ω_{ce}$ and $ω_{pe}$ are the electron cyclotron and plasma frequencies, respectively). In the regime $ω_{ce}/ω_{pe}\lesssim 1.2-1.7$ the instability is dominated by oblique, quasi-electrostatic (OQES) modes, and the acceleration is inefficient. When $ω_{ce}/ω_{pe}$ has grown to $ω_{ce}/ω_{pe}\gtrsim 1.2-1.7$, electrons are efficiently accelerated by the inelastic scattering provided by unstable parallel, electromagnetic z (PEMZ) modes. After $ω_{ce}/ω_{pe}$ reaches $\sim 2$, the electron energy spectra show nonthermal tails that differ between the shearing and compressing cases. In the shearing case, the tail resembles a power-law of index $α_s \sim$ 2.9 plus a high-energy bump reaching $\sim 300$ keV. In the compressing runs, $α_s \sim$ 3.7 with a spectral break above $\sim 500$ keV. This difference can be explained by the different temperature evolutions in these two types of simulations, suggesting a critical role played by the type of anisotropy driving, $ω_{ce}/ω_{pe}$ and the electron temperature in the efficiency of the acceleration.

astro-ph.SR↗

arXiv2021

A Toy Model for the Time-Frequency Structure of Fast Radio Bursts: Implications for the CHIME Burst Dichotomy

We introduce a toy model for the time-frequency structure of fast radio bursts (FRB), 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 which 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 signal-to-noise ratio (SNR) is accumulated. We demonstrate that by varying just a single parameter of the toy model-the power-law index βof the frequency drift rate-one can transform a long (and hence preferentially time-resolved) burst with a narrow time-integrated spectrum into a shorter burst with a broad power-law time-integrated spectrum. We suggest that burst-to-burst diversity in the value of βcould generate the dichotomy between burst duration and frequency-width recently found by CHIME. In shock models, the value of βis related to the radial density profile of 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 one-off FRB sources.

astro-ph.HE↗

arXiv2021

Comptonization by Reconnection Plasmoids in Black Hole Coronae I: Magnetically Dominated Pair Plasma

We perform two-dimensional particle-in-cell simulations of reconnection in magnetically dominated electron-positron plasmas subject to strong Compton cooling. We vary the magnetization $σ\gg1$, 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 X-rays 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 quasi-Maxwellian shape with an effective temperature of $\sim 100$ 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 high-energy tail, which receives $\sim 25$% of the dissipated reconnection power for $σ=10$ and $\sim 40$% for $σ=40$. We complement our particle-in-cell studies with a Monte-Carlo simulation of the transfer of seed soft photons through the reconnection layer, and find the escaping X-ray spectrum. The simulation demonstrates that Comptonization is dominated by the bulk motions in the chain of Compton-cooled plasmoids and, for $σ\sim 10$, yields a spectrum consistent with the typical hard state of accreting black holes.

astro-ph.HE↗

arXiv2021

Pitch Angle Anisotropy Controls Particle Acceleration and Cooling in Radiative Relativistic Plasma Turbulence

Nature's most powerful high-energy sources are capable of accelerating particles to high energy and radiate it away on extremely short timescales, even shorter than the light crossing time of the system. It is yet unclear what physical processes can produce such an efficient acceleration, despite the copious radiative losses. By means of radiative particle-in-cell simulations, we show that magnetically dominated turbulence in pair plasmas subject to strong synchrotron cooling generates a nonthermal particle spectrum with a hard power-law range (slope $p \sim 1$) within a few eddy turnover times. Low pitch-angle particles can significantly exceed the nominal radiation-reaction limit, before abruptly cooling down. The particle spectrum becomes even harder ($p < 1$) over time owing to particle cooling with an energy-dependent pitch-angle anisotropy. The resulting synchrotron spectrum is hard ($νF_ν\propto ν^s$ with $s \sim 1$). Our findings have important implications for understanding the nonthermal emission from high-energy astrophysical sources, most notably the prompt phase of gamma-ray bursts and gamma-ray flares from the Crab nebula.

physics.plasm-ph↗

arXiv2021

Coherent Electromagnetic Emission from Relativistic Magnetized Shocks

Relativistic magnetized shocks are a natural source of coherent emission, offering a plausible radiative mechanism for Fast Radio Bursts (FRBs). We present first-principles 3D simulations that provide essential information for the FRB models based on shocks: the emission efficiency, spectrum, and polarization. The simulated shock propagates in an $e^\pm$ plasma with magnetization $σ>1$. The measured fraction of shock energy converted to coherent radiation is $\simeq 10^{-3} \, σ^{-1}$, and the energy-carrying wavenumber of the wave spectrum is $\simeq 4 \,ω_{\rm c}/c$, where $ω_{\rm c}$ is the upstream gyrofrequency. The ratio of the O-mode and X-mode energy fluxes emitted by the shock is $\simeq 0.4\,σ^{-1}$. The dominance of the X-mode at $σ\gg 1$ is particularly strong, approaching 100% in the spectral band around $2\,ω_{\rm c}$. We also provide a detailed description of the emission mechanism for both X- and O-modes.

astro-ph.HE↗

arXiv2021

Periodic Fast Radio Bursts from Luminous X-ray Binaries

The discovery of periodicity in the arrival times of the fast radio bursts (FRBs) poses a challenge to the oft-studied magnetar scenarios. However, models that postulate that FRBs result from magnetized shocks or magnetic reconnection in a relativistic outflow are not specific to magnetar engines; instead, they require only the impulsive injection of relativistic energy into a dense magnetized medium. Motivated thus, we outline a new scenario in which FRBs are powered by short-lived relativistic outflows (``flares'') from accreting black holes or neutron stars, which propagate into the cavity of the pre-existing (``quiescent'') jet. In order to reproduce FRB luminosities and rates, we are driven to consider binaries of stellar-mass compact objects undergoing super-Eddington mass-transfer, similar to ultraluminous X-ray (ULX) sources. Indeed, the host galaxies of FRBs, and their spatial offsets within their hosts, show broad similarities with ULXs. Periodicity on timescales of days to years could be attributed to precession (e.g., Lens-Thirring) of the polar accretion funnel, along which the FRB emission is geometrically and relativistically beamed, which sweeps across the observer line of sight. Accounting for the most luminous FRBs via accretion power may require a population of binaries undergoing brief-lived phases of unstable (dynamical-timescale) mass-transfer. This will lead to secular evolution in the properties of some repeating FRBs on timescales of months to years, followed by a transient optical/IR counterpart akin to a luminous red nova, or a more luminous accretion-powered optical/X-ray transient. We encourage targeted FRB searches of known ULX sources.

astro-ph.HE↗

arXiv2021

Synchrotron-Self-Compton radiation from magnetically-dominated turbulent plasmas in relativistic jets

Relativistic jets launched by rotating black holes are powerful emitters of non-thermal radiation. Extraction of the rotational energy via electromagnetic stresses produces magnetically-dominated jets, which may become turbulent. Studies of magnetically-dominated plasma turbulence from first principles show that most of the accelerated particles have small pitch angles, i.e. the particle velocity is nearly aligned with the local magnetic field. We examine synchrotron-self-Compton radiation from anisotropic particles in the fast cooling regime. The small pitch angles reduce the synchrotron cooling rate and promote the role of inverse Compton (IC) cooling, which can occur in two different regimes. In the Thomson regime, both synchrotron and IC components have soft spectra, $νF_ν\proptoν^{1/2}$. In the Klein-Nishina regime, synchrotron radiation has a hard spectrum, typically $νF_ν\proptoν$, over a broad range of frequencies. Our results have implications for the modelling of BL Lacs and Gamma-Ray Bursts (GRBs). BL Lacs produce soft synchrotron and IC spectra, as expected when Klein-Nishina effects are minor. The observed synchrotron and IC luminosities are typically comparable, which indicates a moderate anisotropy with pitch angles $θ\gtrsim0.1$. Rare orphan gamma-ray flares may be produced when $θ\ll0.1$. The hard spectra of GRBs may be consistent with synchrotron radiation when the emitting particles are IC cooling in the Klein-Nishina regime, as expected for pitch angles $θ\sim0.1$. Blazar and GRB spectra can be explained by turbulent jets with a similar electron plasma magnetisation parameter, $σ_{\rm e}\sim10^4$, which for electron-proton plasmas corresponds to an overall magnetisation $σ=(m_{\rm e}/m_{\rm p})σ_{\rm e}\sim10$.

astro-ph.HE↗

arXiv2021

Secondary Energization in Compressing Plasmoids during Magnetic Reconnection

Plasmoids -- magnetized quasi-circular structures formed self-consistently in reconnecting current sheets -- were previously considered to be the graveyards of energetic particles. In this paper, we demonstrate the important role of plasmoids in shaping the particle energy spectrum in relativistic reconnection (i.e., with upstream magnetization $σ_{\rm up} \gg 1$). Using two dimensional particle-in-cell simulations in pair plasmas with $σ_{\rm up}=10$ and $100$, we study a secondary particle energization process that takes place inside compressing plasmoids. We demonstrate that plasmoids grow in time, while their interiors compress, amplifying the internal magnetic field. The magnetic field felt by particles injected in an isolated plasmoid increases linearly with time, which leads to particle energization as a result of magnetic moment conservation. For particles injected with a power-law distribution function, this energization process acts in such a way that the shape of the injected power law is conserved, while producing an additional non-thermal tail $f(E)\propto E^{-3}$ at higher energies followed by an exponential cutoff. The cutoff energy, which increases with time as $E_{\rm cut}\propto\sqrt{t}$, can greatly exceed $σ_{\rm up} m_e c^2$. We analytically predict the secondary acceleration timescale and the shape of the emerging particle energy spectrum, which can be of major importance in certain astrophysical systems, such as blazar jets.

astro-ph.HE↗