arXiv subjects

Emanuele Sobacchi

Publications and source records attributed to Emanuele Sobacchi.

At least 19 recordsLinked to original sources

arXiv2026

Interaction of Strong Electromagnetic Waves with Unmagnetized Pair Plasmas

We investigate analytically and numerically the interaction of strong electromagnetic waves with unmagnetized pair plasmas. We show that the interaction is governed by a single nonlinearity parameter, $\varepsilon_{\rm p}$, defined as the ratio of the wave strength parameter to the wave frequency in units of the plasma frequency (with both frequencies measured in the plasma rest frame prior to the interaction). When $\varepsilon_{\rm p}<1$, the number of wavelengths that propagate through the plasma without attenuation from induced Compton scattering is approximately $\varepsilon_{\rm p}^{-2/3}$. This attenuation can imprint sub-structures as narrow as a few wavelengths on the pulse profile. When $\varepsilon_{\rm p}>1$, the electromagnetic pulse acts as a relativistic piston and drives a shock into the plasma. Our results establish a framework for the interaction of strong electromagnetic waves with pair plasmas, a process relevant for intense radio pulses from neutron stars and for next-generation pair plasma experiments at multi-petawatt laser facilities.

physics.plasm-ph↗

arXiv2026

MeV absorption in gamma-ray bursts as a probe of their progenitor environments

A small fraction of X-ray photons from $γ$-ray bursts (GRBs), after escaping the relativistic jet, are scattered by electrons in the circumburst medium. Subsequent photon-photon absorption between the incoming MeV $γ$-rays and the back-scattered X-rays generate electron-positron pairs, enriching the surrounding medium with leptons. We investigate how these back-scattered photons modify the prompt GRB spectrum through $γ-γ$ absorption. In a dense and pair-loaded environment, the emerging spectra exhibit a broad absorption feature, whose profile is sensitive to the low-energy spectral index $α$. In particular, spectra with $α> -1$ develop a pronounced, saddle-shaped absorption between 1 and 100 MeV (rest frame). Such external MeV absorption could account for the spectral curvature seen in some bright GRBs, and may point to a dense circum-stellar medium (CSM) around their progenitor stars - consistent with early observations of core-collapse supernovae. In this scenario, the blastwave caused by the GRB is expected to start off with a relatively low Lorentz factor, and undergo an acceleration phase when traversing the large density drop at the interface between the dense CSM and the surrounding medium. The impact of these non-trivial dynamics on the afterglow emission is yet to be explored.

astro-ph.HE↗

arXiv2026

Acoustic instability at shock-wave precursors

Magnetic field amplification is an integral part of the process of particle acceleration at non-relativistic shocks. It is necessary to reach the maximum energies required by observations, especially in supernova remnants, thought to be sources of the bulk of Galactic cosmic rays. Such amplification can be caused by the acoustic instability that develops when small density perturbations interact with the cosmic-ray pressure gradient in the upstream of a cosmic-ray-modified shock. The vorticity induced by the nonlinear development of the instability may lead to turbulence, which amplifies the pre-existing magnetic fields. To study this phenomenon, we use the PLUTO code to carry out 2D (and some 3D) magnetohydrodynamical simulations of the evolution of small density perturbations in the presence of an assigned cosmic-ray pressure gradient. Adopting more realistic values of Mach number and cosmic-ray acceleration efficiency than previously assumed in the literature, we show that the acoustic instability can transform small density perturbations into large nonlinear structures while the fluid crosses the precursor region of a cosmic-ray-modified shock. We study the power spectrum of turbulent magnetic fluctuations that may be important to scatter particles. We comment on the possible constructive interference between acoustic and non-resonant streaming instabilities. We discuss limitations of previous and current numerical investigations in accessing spatial scales where turbulence is expected to turn nonlinear, and outline perspectives for future investigations.

astro-ph.HE↗

arXiv2026

Temporal variability of polarization in blazars

We investigate the temporal variability of polarization of synchrotron radiation from blazar jets. Multiwavelength observations revealed high-amplitude rotations of the electric vector position angle (EVPA), both in the optical and in the X-rays. More often, the polarization degree and the EVPA show a seemingly erratic variability. To interpret these observations, we present a geometric and deterministic model in which off-axis, compact emitting features (i.e.,~blobs) propagate along the jet with the local velocity of the flow. The dynamics of the blobs is determined by the jet electromagnetic fields, which are calculated self-consistently using an analytical model of magnetically dominated outflows. The jet is axisymmetric, and its electromagnetic fields do not have a turbulent component. We show that the observed polarization is sensitive to the initial spatial configurations of the blobs. For the same jet structure, we observe several remarkably complex polarization patterns, including large EVPA rotations of $180^{\circ}$ or more in both directions and more erratic fluctuations. Simultaneous high-amplitude variations of the polarization degree and the EVPA can coincide with peaks of the observed luminosity. However, seemingly uncorrelated variations are also possible. We discuss the feasibility of constraining the particle acceleration mechanism from multifrequency polarimetric observations.

astro-ph.HE↗

arXiv2025

Absorption of strong electromagnetic waves in magnetized pair plasmas

We discuss synchrotron absorption of a short electromagnetic pulse that propagates in a cold magnetized pair plasma. We show that the pulse can be absorbed when $ω_{\rm B}/a_0< ω< a_0ω_{\rm B}$, where $a_0>1$ is the strength parameter of the pulse, and $ω$ and $ω_{\rm B}$ respectively are the frequency of the wave and the cyclotron frequency in the background magnetic field (all quantities are defined in the reference frame where the particles are at rest before being illuminated by the pulse). The condition $ω_{\rm B}/a_0< ω< a_0ω_{\rm B}$ is essentially a generalization of the cyclotron resonance to strong electromagnetic pulses with $a_0>1$. When $ω_{\rm B}/a_0< ω< a_0ω_{\rm B}$, the propagation of electromagnetic waves in a plasma can be very different with respect to the propagation in vacuum because the wave equation is strongly non-linear. Then it is unclear whether the particles are heated stochastically due to synchrotron absorption, as found by studying the motion of a test particle in the field of a vacuum electromagnetic wave. We discuss implications of our results for constraining emission models of fast radio bursts.

astro-ph.HE↗

arXiv2025

Ultra-long MeV transient from a relativistic jet: a tidal disruption event candidate

On July 2, 2025, the Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-ray space telescope detected three short-duration MeV transients with overlapping sky locations. These events, named as GRB 250702D, B, and E (collectively referred to as DBE), triggered the detector with delays of approximately 1-2 hours between each burst. Follow-up observations of this unusually long MeV transient (lasting >3 hours) by the Neil Gehrels Swift Observatory and the Nuclear Spectroscopic Telescope Array over a period of 10 days revealed a steep temporal decline in soft X-rays ($\propto t^{-1.9 \pm 0.1}$). The time-averaged spectra during the outbursts are well described by a single power law $dN_γ/dE \propto E^{-1.5}$, while upper limits above 100 MeV imply a spectral cutoff between 10 MeV and 100 MeV. Using standard gamma-ray transparency arguments, we derive a lower limit on the bulk Lorentz factor. Combined with the steep decline in X-rays, these constraints point to a relativistic jet origin. The properties of DBE are inconsistent with established GRB spectral-energy correlations, disfavoring classical long GRB progenitors. Instead, the basic characteristics of DBE resemble those of previously reported jetted tidal disruption events (TDEs), though alternative progenitor channels cannot be excluded. In the relativistic TDE scenario, DBE is the first one with detected MeV gamma-ray emission. We argue that the observed emission is most likely produced by synchrotron radiation from sub-TeV electrons.

astro-ph.HE↗

arXiv2025

The Role of Acoustic Instability in Cosmic-Ray Self-Confinement

Over the past decades, there has been growing observational and theoretical evidence that cosmic-ray-induced instabilities play an important role in both acceleration and transport of cosmic rays (CRs). For instance, the efficient acceleration of charged particles at supernova remnant shocks requires rapidly growing instabilities, so much so that none of the proposed processes seem sufficient to warrant acceleration to PeV energies. In this work, we investigate whether an acoustic instability triggered by the presence of a CR pressure gradient can lead to significant self-confinement of charged particles in the vicinity of shocks. We validate the expected growth rates and obtain the scale and energy of magnetic field perturbations induced by such system using magnetohydrodynamical simulations. Our results suggest a strong suppression of the diffusion coefficient for particles with Larmor radius around a thousandth of the precursor scale length.

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.

astro-ph.HE↗

arXiv2024

Polarization of synchrotron radiation from blazar jets

Supermassive black holes in active galactic nuclei (AGNs) launch relativistic jets that shine through the entire electromagnetic spectrum. Blazars are a subclass of AGN where non-thermal radiation from the jet is strongly beamed, as the jet is directed nearly toward the observer. Multifrequency polarimetry is emerging as a powerful probe of blazar jets, especially with the advent of the Imaging X-ray Polarimetry Explorer (IXPE) space observatory. IXPE mostly targeted high synchrotron peaked (HSP) blazars, where both optical and X-ray emission can be attributed to synchrotron radiation from a population of non-thermal electrons. Observations of HSP blazars show that the polarization degree is strongly chromatic ($Π_{\rm X}/Π_{\rm O} \sim 2-7$), whereas the electric vector position angle (EVPA) is nearly independent of the observed frequency ($Ψ_{\rm X}\simeqΨ_{\rm O}$). The strong chromaticity of the polarization degree was interpreted as an evidence that non-thermal electrons are accelerated by shocks. We present an alternative scenario that naturally explains IXPE observations. We study the polarization of synchrotron radiation from stationary axisymmetric jets viewed nearly on-axis. We show that the polarization degree increases significantly at high photon frequencies, as the distribution of the emitting electrons becomes softer, whereas the EVPA is nearly constant. The chromaticity of the polarization degree is much stronger in axisymmetric jets than in the case of a uniform magnetic field. Our results show that the topology of the electromagnetic fields is key to interpret multifrequency polarimetric observations of blazar jets. On the other hand, these observations may be less sensitive than previously thought to the specific particle acceleration process (e.g., shocks or magnetic reconnection).

astro-ph.HE↗

arXiv2024

Escape of fast radio bursts from magnetars

Fast radio bursts (FRBs) are bright extragalactic transients likely produced by magnetars. We study the propagation of FRBs in magnetar winds, assuming that the wind is strongly magnetized and composed of electron-positron pairs. We focus on the regime where the strength parameter of the radio wave, $a_0$, is larger than unity, and the wave frequency, $ω_0$, is larger than the Larmor frequency in the background magnetic field, $ω_{\rm L}$. We show that strong radio waves with $a_0>1$ are able to propagate when $ω_0 > a_0ω_{\rm L}$, as the plasma current is a linear function of the wave electric field. The dispersion relation is independent of the wave strength parameter when $ω_0 > a_0ω_{\rm L}$. Instead, radio waves could be damped when $ω_0 < a_0ω_{\rm L}$, as a significant fraction of the wave energy is used to compress the plasma and amplify the background magnetic field. Our results suggest that FRBs should be produced at large distances from the magnetar (i.e., $R>10^{12}{\rm\; cm}$, where the condition $ω_0 > a_0ω_{\rm L}$ is satisfied). Alternatively, the structure of the magnetar wind should be strongly modified during a flare to allow the escape of FRBs produced at radii $R<10^{12}{\rm\; cm}$.

astro-ph.HE↗

arXiv2024

Propagation of strong electromagnetic waves in tenuous plasmas

We study the propagation of electromagnetic waves in tenuous plasmas, where the wave frequency, $ω_0$, is much larger than the plasma frequency, $ω_{\rm P}$. We show that in pair plasmas nonlinear effects are weak for $a_0 \ll ω_0/ω_{\rm P}$, where $a_0$ is the wave strength parameter. In electron-proton plasmas a more restrictive condition must be satisfied, namely either $a_0\ll 1/ω_{\rm P}τ_0$, where $τ_0$ is the duration of the radiation pulse, or $a_0\ll 1$. We derive the equations that govern the evolution of the pulse in the weakly nonlinear regime. Our results have important implications for the modeling of fast radio bursts. We argue that: (i) Millisecond duration bursts with a smooth profile must be produced in a proton-free environment, where nonlinear effects are weaker. (ii) Propagation through an electron-proton plasma near the source can imprint a sub-microsecond variability on the burst profile.

astro-ph.HE↗

arXiv2024

Nuclear rings are the inner edge of a gap around the Lindblad Resonance

Gaseous nuclear rings are large-scale coherent structures commonly found at the centres of barred galaxies. We propose that they are an accumulation of gas at the inner edge of an extensive gap that forms around the Inner Lindblad Resonance (ILR). The gap initially opens because the bar potential excites strong trailing waves near the ILR, which remove angular momentum from the gas disc and transport the gas inwards. The gap then widens because the bar potential continuously excites trailing waves at the inner edge of the gap, which remove further angular momentum, moving the edge further inwards until it stops at a distance of several wavelengths from the ILR. The gas accumulating at the inner edge of the gap forms the nuclear ring. The speed at which the gap edge moves and its final distance from the ILR strongly depend on the sound speed, explaining the puzzling dependence of the nuclear ring radius on the sound speed in simulations.

astro-ph.GA↗

arXiv2023

Kinetic Simulations of the Filamentation Instability in Pair Plasmas

The nonlinear interaction between electromagnetic waves and plasmas attracts significant attention in astrophysics because it can affect the propagation of Fast Radio Bursts (FRBs) -- luminous millisecond-duration pulses detected at radio frequency. The filamentation instability (FI) -- a type of nonlinear wave-plasma interaction -- is considered to be dominant near FRB sources, and its nonlinear development may also affect the inferred dispersion measure of FRBs. In this paper, we carry out fully kinetic particle-in-cell simulations of the FI in unmagnetized pair plasmas. Our simulations show that the FI generates transverse density filaments, and that the electromagnetic wave propagates in near vacuum between them, as in a waveguide. The density filaments keep merging until force balance between the wave ponderomotive force and the plasma pressure gradient is established. We estimate the merging timescale and discuss the implications of filament merging for FRB observations.

astro-ph.HE↗

arXiv2023

Ultrafast Variability in AGN Jets: Intermittency and Lighthouse Effect

Gamma-ray flares from Active Galactic Nuclei (AGN) show substantial variability on ultrafast timescales (i.e. shorter than the light crossing time of the AGN's supermassive black hole). We propose that ultrafast variability is a byproduct of the turbulent dissipation of the jet Poynting flux. Due to the intermittency of the turbulent cascade, the dissipation is concentrated in a set of reconnecting current sheets. Electrons energised by reconnection have a strong pitch angle anisotropy, i.e. their velocity is nearly aligned with the guide magnetic field. Then each current sheet produces a narrow radiation beam, which dominates the emission from the whole jet when it is directed towards the observer. The ultrafast variability is set by the light crossing time of a single current sheet, which is much shorter than the light crossing time of the whole emission region. The predictions of our model are: (i) The bolometric luminosity of ultrafast AGN flares is dominated by the inverse Compton (IC) emission, as the lower energy synchrotron emission is suppressed due to the pitch angle anisotropy. (ii) If the observed luminosity includes a non-flaring component, the variations of the synchrotron luminosity have a small amplitude. (iii) The synchrotron and IC emission are less variable at lower frequencies, as the cooling time of the radiating particles exceeds the light crossing time of the current sheet. Simultaneous multiwavelength observations of ultrafast AGN flares can test these predictions.

astro-ph.HE↗

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

The Physical Origin and the Properties of Arm Spurs/Feathers in Local Simulations of the Wiggle Instability

Gaseous substructures such as feathers and spurs dot the landscape of spiral arms in disc galaxies. One of the candidates to explain their formation is the wiggle instability of galactic spiral shocks. We study the wiggle instability using local 2D hydrodynamical isothermal non-self gravitating simulations. We find that: (1) Simulations agree with analytic linear stability analysis only under stringent conditions. They display surprisingly strong non-linear coupling between the different modes, even for small mode amplitudes ($\sim 1\%$). (2) We demonstrate that the wiggle instability originates from a combination of two physically distinct mechanisms: the first is the Kelvin-Helmholtz instability, and the second is the amplification of infinitesimal perturbations from repeated shock passages. These two mechanisms can operate simultaneously, and which mechanism dominates depends on the underlying parameters. (3) We explore the parameter space and study the properties of spurs/feathers generated by the wiggle instability. The wiggle instability is highly sensitive to the underlying parameters. The feather separation decreases, and the growth rate increases, with decreasing sound speed, increasing potential strength and decreasing interarm distance. (4) We compare our simulations with a sample of 20 galaxies in the HST Archival Survey of Spiral Arm Substructure of La Vigne et al. and find that the wiggle instability is able to reproduce the typical range of feather separations seen in observations. It remains unclear how the wiggle instability relates to competing mechanisms for spur/feather formation such as the magneto-jeans instability and the stochastic accumulation of gas due to correlated supernova feedback.

astro-ph.GA↗

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

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↗