arXiv subjects

Luca Del Zanna

Publications and source records attributed to Luca Del Zanna.

At least 19 recordsLinked to original sources

arXiv2026

The impact of nuclear equations of state on the dynamics and multi-messenger emission of magnetorotational stellar explosions

The gravitational collapse of massive stars at the end of their life leads to powerful supernova explosions that produce compact objects, regulate the dynamics of host galaxies, and contribute to cosmic chemical evolution. In the presence of fast rotation and strong magnetic fields, such explosions can reach extreme energies, explaining sources such as hypernovae and long gamma-ray bursts. We investigate the impact of variations in the nuclear equation of state (EoS) on magnetorotational explosions and their multimessenger emission, including neutrinos and gravitational waves. Differences in stiffness, composition, and finite-temperature behavior of the EoS affect the collapse, bounce, and jet-launching phases. Using the Aenus-Alcar code, which includes relativistic magnetohydrodynamics, two-moment neutrino transport, neutrino-matter interactions, and general-relativistic corrections, we perform axisymmetric simulations with different EoSs. All models start from the same pre-supernova progenitor with solar metallicity, a zero-age main sequence mass of 20 solar masses, a dipolar magnetic field, and a shellular rotation profile. The different EoSs produce significant variations in explosion dynamics, proto-neutron star properties, ejecta mass, and multimessenger signals. Our results show that magnetorotational core-collapse supernova signatures depend not only on the cold stiffness of the EoS, but also on its thermal and compositional properties, highlighting the importance of combining gravitational-wave and neutrino observations to constrain dense matter physics and the explosion mechanism.

astro-ph.HE↗

arXiv2026

2D or not 2D? Exploring 3D relativistic magnetic reconnection dynamics with highly accurate numerical simulations

Fast reconnection in magnetically dominated plasmas is widely invoked in models of dissipation in pulsar winds, gamma-ray flares in the Crab nebula, and to explain the radio nanoshots of pulsars. When current sheets evolve reaching a critical inverse aspect ratio, scaling as $S^{-1/3}$ with the plasma Lundquist number, the so-called \textit{ideal} tearing instability sets in, with modes growing, independently of $S$, extremely rapidly on timescales of only a few light-crossing times of the sheet length. We present the first set of fully 3D simulations of current-sheet disruption triggered by the ideal tearing instability within the resistive relativistic MHD approximation, as appropriate in situations where the Alfvén velocity approaches the speed of light. We compare 3D setups with different initial conditions with their 2D counterparts, and we assess the impact of dimensionality and of the magnetic field topology on the onset, evolution, and efficiency of reconnection. In force-free configurations, 3D runs develop ideal tearing, secondary instabilities, and a thick, turbulent current layer, sustaining dissipation of magnetic energy longer than in 2D. In pressure-balanced current sheets with a null guide field, 2D reference runs show the familiar reconnection dynamics, whereas in 3D tearing dynamics is quenched after the linear phase, as pressure-driven modes growing on forming plasmoids outcompete plasmoid coalescence and suppress fast dissipation of magnetic energy. Taken together, these results suggest that the evolution and efficiency of reconnection depend sensitively on the local plasma conditions and current-sheet configuration, and can be properly captured only in fully 3D simulations.

astro-ph.HE↗

arXiv2025

Polarization properties of synchrotron sources from simulations of relativistic magnetohydrodynamic turbulence

The emission from the relativistically hot plasmas of high-energy astrophysical synchrotron sources, pulsar wind nebulae (PWNe) in particular, depends on the level of magnetic fluctuations. Recent observations by the X-ray polarimeter IXPE support the presence of turbulence, with varying conditions even in different regions of a same source. We model such emission, and in particular the degree of linear polarization, by using 3D relativistic magnetohydrodynamic (MHD) turbulence simulations for the first time. Thanks to a novel accelerated version of the ECHO code, a series of 3D relativistic MHD simulations were performed assuming a relativistically hot plasma and various degrees of magnetization, mimicking different conditions encountered in synchrotron sources. Magnetic fluctuations at random directions with respect to a background field were initialized at large scales. After the full development of the turbulent cascade, the statistical properties of the plasma and of the synchrotron emission maps were analyzed. Turbulence rapidly relaxes to a sort of Alfvénic equilibrium and a Kolmogorov cascade with a slope of $-5/3$ soon develops, with differences depending on the initial ratio, $η$, of magnetic fluctuations over the background field. Dissipation mostly occurs in thin current sheets, where (numerical) reconnection takes place and intermittency and deviation from isotropic Gaussian distributions are observed. Synthetic synchrotron maps and their statistical properties depend on $η$ too, approaching analytical estimates for large $η$. The integrated degree of linear polarization is found to cover the whole range of observed values in PWNe, and its dependence on the relative amplitude of turbulent fluctuations shows a good agreement with analytical estimates, even in the presence of anisotropy.

astro-ph.HE↗

arXiv2025

Electric conductivity and flavor diffusion in a viscous, resistive quark-gluon plasma for weak and strong magnetic fields

We present a microscopic calculation of the electric conductivity and net-particle diffusion coefficients for a viscous and resistive ultra-relativistic plasma. Our results might be of interest for several astrophysical and cosmological problems, but the main physical application we have in mind is the hot deconfined matter produced in relativistic heavy-ion collisions. Accordingly, as charged particles of the medium we take three species (flavors) of light (massless for the sake of simplicity) quarks -- $u$, $d$ and $s$ -- and antiquarks, entailing the existence of three macroscopic conserved charges: baryon number ${\cal B}$, electric charge $Q$ and strangeness $S$. Our results are valid both in a weakly and in a strongly-magnetized plasma, where the energy stored in the magnetic field is comparable to the one carried by the medium particles. Actually, for a conformal fluid, the behavior of the system only depends on the ratio between the thermal and the magnetic pressure, the so-called plasma beta-parameter, acting as a scaling variable. Our calculation, starting from a relativistic Boltzmann-Vlasov equation, is based on a generalized Chapman-Enskog approach in which space-time gradients and the local electric field are treated as first-order quantities in a perturbative expansion, while terms containing magnetic corrections are considered of zeroth order and hence self-consistently resummed. We find that, also in the strong-field limit, for each conserved charge a generalized Wiedemann-Franz law, connecting charge conductivity and diffusion coefficient, exists. However these transport coefficients acquire a non-trivial tensor structure, reflecting the development of a longitudinal, a transverse and a Hall current as a response to electric fields or density gradients.

hep-ph↗

arXiv2024

Magnetic dissipation in short gamma-ray burst jets. I. Resistive relativistic MHD evolution in a model environment

Short gamma-ray bursts originate when relativistic jets emerge from the remnants of binary neutron star mergers. Both the jet and the remnant are believed to be strongly magnetized, and the presence of magnetic fields is known to influence the jet propagation across the surrounding post-merger environment. In the magnetic interplay between the jet and the environment itself, effects due to a finite plasma conductivity may be important, especially in the first phases of the jet propagation. We aim to investigate such effects, from jet launching to its final breakout from the post-merger environment. 2D axisymmetric and full 3D resistive relativistic MHD simulations, are performed with the PLUTO numerical code. Different models for physical resistivity, which must be small but still above the numerical one (producing unwanted smearing of structures in any ideal MHD code) are considered and compared. All simulations are performed by using an axisymmetric analytical model for the jet propagation environment; we leave the case of jet propagation in a realistic environment (i.e. imported from actual binary neutron star merger simulation) to a later study. Significant differences in the jet structure and induced turbulence are clearly seen in 2D axisymmetric simulations. Regions with a resistive electric field parallel to the magnetic field form and non-thermal particle acceleration may be enhanced there. The level of dissipated Ohmic power is also dependent on the various recipes for resistivity. Most of the differences arise before breakout from the magnetized environment, whereas once the jet enters the external atmosphere these differences are preserved during further propagation despite the lower grid refinement. Finally, we show and discuss the 3D evolution of the jet within the same environment, in order to highlight the emergence of non-axisymmetric features.

astro-ph.HE↗

arXiv2024

A Fourth-Order Finite Volume Scheme for Resistive Relativistic Magnetohydrodynamics

We present a finite-volume, genuinely 4th-order accurate numerical method for solving the equations of resistive relativistic magnetohydrodynamics (Res-RMHD) in Cartesian coordinates. In our formulation, the magnetic field is evolved in time in terms of face-average values via the constrained-transport method while the remaining variables (density, momentum, energy and electric fields) are advanced as cell volume-averages. Spatial accuracy employs 5th-order accurate WENO-Z reconstruction from point values (as described in a companion paper) to obtain left and right states at zone interfaces. Explicit flux evaluation is carried out by solving a Riemann problem at cell interfaces, using the Maxwell-Harten-Lax-van Leer with contact wave resolution (MHLLC). Time stepping is based on the implicit-explicit (IMEX) Runge-Kutta (RK) methods, of which we consider both the 3rd-order strong stability preserving SSP3(4,3,3) and a recent 4th-order additive RK scheme, to cope with the stiffness introduced by the source term in Ampere's law. Numerical benchmarks are presented in order to assess the accuracy and robustness of our implementation.

astro-ph.HE↗

arXiv2024

A GPU-Accelerated Modern Fortran Version of the ECHO Code for Relativistic Magnetohydrodynamics

The numerical study of relativistic magnetohydrodynamics (MHD) plays a crucial role in high-energy astrophysics, but unfortunately is computationally demanding, given the complex physics involved (high Lorentz factor flows, extreme magnetization, curved spacetimes near compact objects) and the large variety of spatial scales needed to resolve turbulent motions. A great benefit comes from the porting of existing codes running on standard processors to GPU-based platforms. However, this usually requires a drastic rewriting of the original code, the use of specific languages like CUDA, and a complex analysis of data management and optimization of parallel processes. Here we describe the porting of the ECHO code for special and general relativistic MHD to accelerated devices, simply based on native Fortran language built-in constructs, especially 'do concurrent' loops, few OpenACC directives, and the straightforward data management provided by the Unified Memory option of NVIDIA compilers.Thanks to these very minor modifications to the original code, the new version of ECHO runs at least 16 times faster on GPU platforms compared to CPU-based ones. The chosen benchmark is the 3D propagation of a relativistic MHD Alfvén wave, for which strong and weak scaling tests performed on the LEONARDO pre-exascale supercomputer at CINECA are provided (using up to 256 nodes corresponding to 1024 GPUs, and over 14 billion cells). Finally, an example of high-resolution relativistic MHD Alfvénic turbulence simulation is shown, demonstrating the potential for astrophysical plasmas of the new GPU-based version of ECHO.

astro-ph.HE↗

arXiv2023

Resistive relativistic MHD simulations of astrophysical jets

Aims. The main goal of the present paper is to provide the first systematic numerical study of the propagation of astrophysical relativistic jets, in the context of high-resolution shock-capturing resistive relativistic magnetohydrodynamics (RRMHD) simulations. We aim at investigating different values and models for the plasma resistivity coefficient, and at assessing their impact on the level of turbulence, the formation of current sheets and reconnection plasmoids, the electromagnetic energy content, and the dissipated power. Methods. We use the PLUTO code for simulations and we assume an axisymmetric setup for jets, endowed with both poloidal and toroidal magnetic fields, and propagating in a uniform magnetized medium. The gas is assumed to be characterized by a realistic Synge-like equation of state (Taub equation), appropriate for such type of astrophysical jets. The Taub equation is combined here for the first time with the Implicit-Explicit Runge-Kutta time-stepping procedure, as required in RRMHD simulations. Results. The main result is that turbulence is clearly suppressed for the highest values of resistivity (low Lundquist numbers), current sheets are broader, and plasmoids are barely present, while for low values of resistivity results are very similar to ideal runs, where dissipation is purely numerical. We find that recipes employing a variable resistivity based on the advection of a jet tracer or on the assumption of a uniform Lundquist number improve on the use of a constant coefficient and are probably more realistic, preserving the development of turbulence and of sharp current sheets, possible sites for the acceleration of the non-thermal particles producing the observed high-energy emission.

astro-ph.HE↗

arXiv2021

Are GRMHD Mean-Field Dynamo Models of Thick Accretion Disks SANE?

The remarkable results by the Event Horizon Telescope collaboration concerning the emission from M87* and, more recently, its polarization properties, require an increasingly accurate modeling of the plasma flows around the accreting black hole. Radiatively inefficient sources such as M87* and Sgr A* are typically modeled with the SANE (standard and normal evolution) paradigm, if the accretion dynamics is smooth, or with the MAD (magnetically arrested disk) paradigm, if the black hole's magnetosphere reacts by halting the accretion sporadically, resulting in a highly dynamical process. While the recent polarization studies seem to favor MAD models, this may not be true for all sources, and SANE accretion surely still deserves attention. In this work, we investigate the possibility of reaching the typical degree of magnetization and other accretion properties expected for SANE disks by resorting to the mean-field dynamo process in axisymmetric GRMHD simulations, which are supposed to mimic the amplifying action of an unresolved magnetorotational instability-driven turbulence. We show that it is possible to reproduce the main diagnostics present in the literature by starting from very unfavorable initial configurations, such as a purely toroidal magnetic field with negligible magnetization.

astro-ph.HE↗

arXiv2021

Axisymmetric equilibrium models of magnetised neutron stars in scalar-tensor theories

General relativity probably is not the definitive theory of gravity, due a number or issues, both from the theoretical and from the observational point of view. Alternative theories of gravity were conceived to extend general relativity and account for such issues. Among the most promising ones are scalar-tensor theories, which predict an enrichment of the phenomenology of compact objects, like neutron stars. We updated the well-tested XNS code to numerically solve the Einstein-Maxwell equations for a stationary, magnetised neutron star in a class of scalar-tensor theories containing the spontaneous scalarisation phenomenon. We found that there exist "quasi-universal relations" among the mass, radius, scalar charge and magnetic deformation of a neutron star that are true independently of the equation of state, both in general relativity and in scalar-tensor theories. This result could potentially provide new tools to test scalar-tensor theories and the magnetic field geometry inside neutron stars.

astro-ph.HE↗

arXiv2020

Systematic construction of upwind constrained transport schemes for MHD

The constrained transport (CT) method reflects the state of the art numerical technique for preserving the divergence-free condition of magnetic field to machine accuracy in multi-dimensional MHD simulations performed with Godunov-type, or upwind, conservative codes. The evolution of the different magnetic field components, located at zone interfaces using a staggered representation, is achieved by calculating the electric field components at cell edges, in a way that has to be consistent with the Riemann solver used for the update of cell-centered fluid quantities at interfaces. Albeit several approaches have been undertaken, the purpose of this work is, on the one hand, to compare existing methods in terms of robustness and accuracy and, on the other, to extend the \emph{upwind contrained transport} (UCT) method by Londrillo \& Del Zanna (2004) and Del Zanna et al. (2007) for the systematic construction of new averaging schemes. In particular, we propose a general formula for the upwind fluxes of the induction equation which simply involves the information available from the base Riemann solver employed for the fluid part, provided it does not require full spectral decomposition, and 1D reconstructions of velocity and magnetic field components from nearby intercell faces to cell edges. Our results are presented here in the context of second-order schemes for classical MHD, but they can be easily generalized to higher than second order schemes, either based on finite volumes or finite differences, and to other physical systems retaining the same structure of the equations, such as that of relativistic or general relativistic MHD.

physics.comp-ph↗

arXiv2020

Magnetic fields in heavy ion collisions: flow and charge transport

At the earliest times after a heavy-ion collision, the magnetic field created by the spectator nucleons will generate an extremely strong, albeit rapidly decreasing in time, magnetic field. The impact of this magnetic field may have detectable consequences, and is believed to drive anomalous transport effects like the Chiral Magnetic Effect (CME). We detail an exploratory study on the effects of a dynamical magnetic field on the hydrodynamic medium created in the collisions of two ultrarelativistic heavy-ions, using the framework of numerical ideal MagnetoHydroDynamics (MHD) with the ECHO-QGP code. In this study, we consider a magnetic field captured in a conducting medium, where the conductivity can receive contributions from the electromagnetic conductivity $σ$ and the chiral magnetic conductivity $σ_χ$. We first study the elliptic flow of pions, which we show is relatively unchanged by the introduction of a magnetic field. However, by increasing the magnitude of the magnetic field, we find evidence for an enhancement of the elliptic flow in peripheral collisions. Next, we explore the impact of the chiral magnetic conductivity on electric charges produced at the edges of the fireball. This initial $σ_χ$ can be understood as a long-wavelength effective description of chiral fermion production. We then demonstrate that this chiral charge, when transported by the MHD medium, produces a charge dipole perpendicular to the reaction plane which extends a few units in rapidity. Assuming charge conservation at the freeze-out surface, we show that the produced charge imbalance can have measurable effects on some experimental observables, like $v_1$ or $\langle \sin ϕ\rangle$. This demonstrates the ability of a MHD fluid to transport the signature of the initial chiral magnetic fields to late times.

hep-ph↗

arXiv2019

Fast Magnetic Reconnection: Secondary Tearing Instability and Role of the Hall Term

Magnetic reconnection provides the primary source for explosive energy release, plasma heating and particle acceleration in many astrophysical environments. The last years witnessed a revival of interest in the MHD tearing instability as a driver for efficient reconnection. It has been established that, provided the current sheet aspect ratio becomes small enough ($a/L \sim S^{-1/3}$ for a given Lundquist number $S\gg 1$), reconnection occurs on ideal Alfvén timescales and becomes independent on $S$. Here we investigate, by means of two-dimensional simulations, the \emph{ideal} tearing instability in the Hall-MHD regime, which is appropriate when the width of the resistive layer $δ$ becomes comparable to the ion inertial length $d_i$. Moreover, we study in detail the spontaneous development and reconnection of secondary current sheets, which for high $S$ naturally adjust to the ideal aspect ratio and hence their evolution proceeds very rapidly. For moderate low $S$, the aspect ratio tends to the Sweet-Parker scaling ($a/L \sim S^{-1/2}$), in order to fulfill the condition $δ\ll a$ necessary for the onset of a tearing instability. When the Hall term is included, the reconnection rate of this secondary nonlinear phase is enhanced and, depending on the ratio $d_i/δ$, can be twice with respect to the pure MHD case, and up to ten times larger than the linear phase. Therefore, the evolution of the tearing instability in thin current sheets in the Hall-MHD regime naturally leads to an explosive disruption of the reconnecting site and to energy release on super-Alfvénic timescales, as required to explain astrophysical observations.

physics.plasm-ph↗

arXiv2019

The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project

Recent developments in compact object astrophysics, especially the discovery of merging neutron stars by LIGO, the imaging of the black hole in M87 by the Event Horizon Telescope (EHT) and high precision astrometry of the Galactic Center at close to the event horizon scale by the GRAVITY experiment motivate the development of numerical source models that solve the equations of general relativistic magnetohydrodynamics (GRMHD). Here we compare GRMHD solutions for the evolution of a magnetized accretion flow where turbulence is promoted by the magnetorotational instability from a set of nine GRMHD codes: Athena++, BHAC, Cosmos++, ECHO, H-AMR, iharm3D, HARM-Noble, IllinoisGRMHD and KORAL. Agreement between the codes improves as resolution increases, as measured by a consistently applied, specially developed set of code performance metrics. We conclude that the community of GRMHD codes is mature, capable, and consistent on these test problems.

astro-ph.HE↗

arXiv2018

Covariant and 3+1 Equations for Dynamo-Chiral General Relativistic Magnetohydrodynamics

The exponential amplification of initial seed magnetic fields in relativistic plasmas is a very important topic in astrophysics, from the conditions in the early Universe to the interior of neutron stars. While dynamo action in a turbulent plasma is often invoked, in the last years a novel mechanism of quantum origin has gained increasingly more attention, namely the Chiral Magnetic Effect (CME). This has been recognized in semi-metals and it is most likely at work in the quark-gluon plasma formed in heavy-ion collision experiments, where the highest magnetic fields in nature, up to B~10^18 G, are produced. This effect is expected to survive even at large hydrodynamical/MHD scales and it is based on the chiral anomaly due to an imbalance between left- and right-handed relativistic fermions in the constituent plasma. Such imbalance leads to an electric current parallel to an external magnetic field, which is precisely the same mechanism of an alpha-dynamo action in classical MHD. Here we extend the close parallelism between the chiral and the dynamo effects to relativistic plasmas and we propose a unified, fully covariant formulation of the generalized Ohm's law. Moreover, we derive for the first time the 3+1 general relativistic MHD equations for a chiral plasma both in flat and curved spacetimes, in view of numerical investigation of the CME in compact objects, especially magnetars, or of the interplay among the non-ideal magnetic effects of dynamo, the CME and reconnection.

astro-ph.HE↗

arXiv2018

Fast magnetic reconnection: The "ideal" tearing instability in classic, Hall, and relativistic plasmas

Magnetic reconnection is believed to be the driver of many explosive phenomena in Astrophysics, from solar to gamma-ray flares in magnetars and in the Crab nebula. However, reconnection rates from classic MHD models are far too slow to explain such observations. Recently, it was realized that when a current sheet gets sufficiently thin, the reconnection rate of the tearing instability becomes "ideal", in the sense that the current sheet destabilizes on the "macroscopic" Alfvénic timescales, regardless of the Lundquist number of the plasma. Here we present 2D compressible MHD simulations in the classical, Hall, and relativistic regimes. In particular, the onset of secondary tearing instabilities is investigated within Hall-MHD for the first time. In the frame of relativistic MHD, we summarize the main results from Del Zanna et al. [1]: the relativistic tearing instability is found to be extremely fast, with reconnection rates of the order of the inverse of the light crossing time, as required to explain the high-energy explosive phenomena.

physics.plasm-ph↗

arXiv2017

Papaloizou-Pringle instability suppression by the magnetorotational instability in relativistic accretion discs

Geometrically thick tori with constant specific angular momentum have been widely used in the last decades to construct numerical models of accretion flows onto black holes. Such discs are prone to a global non-axisymmetric hydrodynamic instability, known as Papaloizou-Pringle instability (PPI), which can redistribute angular momentum and also lead to an emission of gravitational waves. It is, however, not clear yet how the development of the PPI is affected by the presence of a magnetic field and by the concurrent development of the magnetorotational instability (MRI). We present a numerical analysis using three-dimensional GRMHD simulations of the interplay between the PPI and the MRI considering, for the first time, an analytical magnetized equilibrium solution as initial condition. In the purely hydrodynamic case, the PPI selects as expected the large-scale $m=1$ azimuthal mode as the fastest growing and non-linearly dominant mode. However, when the torus is threaded by a weak toroidal magnetic field, the development of the MRI leads to the suppression of large-scale modes and redistributes power across smaller scales. If the system starts with a significantly excited $m=1$ mode, the PPI can be dominant in a transient phase, before being ultimately quenched by the MRI. Such dynamics may well be important in compact star mergers and tidal disruption events.

astro-ph.HE↗

arXiv2017

Multidimensional relativistic MHD simulations of Pulsar Wind Nebulae: dynamics and emission

Pulsar Wind Nebulae, and the Crab nebula in particular, are the best cosmic laboratories to investigate the dynamics of magnetized relativistic outflows and particle acceleration up to PeV energies. Multidimensional MHD modeling by means of numerical simulations has been very successful at reproducing, to the very finest details, the innermost structure of these synchrotron emitting nebulae, as observed in the X-rays. Therefore, the comparison between the simulated source and observations can be used as a powerful diagnostic tool to probe the physical conditions in pulsar winds, like their composition, magnetization, and degree of anisotropy. However, in spite of the wealth of observations and of the accuracy of current MHD models, the precise mechanisms for magnetic field dissipation and for the acceleration of the non-thermal emitting particles are mysteries still puzzling theorists to date. Here we review the methodologies of the computational approach to the modeling of Pulsar Wind Nebulae, discussing the most relevant results and the recent progresses achieved in this fascinating field of high-energy astrophysics.

astro-ph.HE↗