arXiv subjects

Luca Del Zanna

Publications and source records attributed to Luca Del Zanna.

32 records · Page 2Linked to original sources

arXiv2017

Numerical magneto-hydrodynamics for relativistic nuclear collisions

We present an improved version of the ECHO-QGP numerical code, which self-consistently includes for the first time the effects of electromagnetic fields within the framework of relativistic magnetohydrodynamics (RMHD). We discuss results of its application in relativistic heavy-ion collisions in the limit of infinite electrical conductivity of the plasma. After reviewing the relevant covariant $3\!+\!1$ formalisms, we illustrate the implementation of the evolution equations in the code and show the results of several tests aimed at assessing the accuracy and robustness of the implementation. After providing some estimates of the magnetic fields arising in non-central high-energy nuclear collisions, we perform full RMHD simulations of the evolution of the Quark-Gluon Plasma in the presence of electromagnetic fields and discuss the results. In our ideal RMHD setup we find that the magnetic field developing in non-central collisions does not significantly modify the elliptic-flow of the final hadrons. However, since there are uncertainties in the description of the pre-equilibrium phase and also in the properties of the medium, a more extensive survey of the possible initial conditions as well as the inclusion of dissipative effects are indeed necessary to validate this preliminary result.

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arXiv2015

Resistive magnetohydrodynamic simulations of the ideal tearing mode

We study the linear and nonlinear evolution of the tearing instability on thin current sheets by means of two-dimensional numerical simulations, within the framework of compressible, resistive magnetohydrodynamics. In particular we analyze the behavior of current sheets whose inverse aspect ratio scales with the Lundquist number $S$ as $S^{-1/3}$. This scaling has been recently recognized to yield the threshold separating fast, ideal reconnection, with an evolution and growth which are independent of $S$ provided this is high enough, as it should be natural having the ideal case as a limit for $S\to\infty$. Our simulations confirm that the tearing instability growth rate can be as fast as $γ\approx 0.6\,{τ_A}^{-1}$, where $τ_A$ is the ideal Alfvénic time set by the macroscopic scales, for our least diffusive case with $S=10^7$. The expected instability dispersion relation and eigenmodes are also retrieved in the linear regime, for the values of $S$ explored here. Moreover, in the nonlinear stage of the simulations we observe secondary events obeying the same critical scaling with $S$, here calculated on the \emph{local}, much smaller lengths, leading to increasingly faster reconnection. These findings strongly support the idea that in a fully dynamic regime, as soon as current sheets develop, thin and reach this critical threshold in their aspect ratio, the tearing mode is able to trigger plasmoid formation and reconnection on the local (ideal) Alfvénic timescales, as required to explain the explosive flaring activity often observed in solar and astrophysical plasmas.

astro-ph.SR↗

arXiv2015

General relativistic radiation hydrodynamics of accretion flows. I: Bondi-Hoyle accretion

We present a new code for performing general-relativistic radiation-hydrodynamics simulations of accretion flows onto black holes. The radiation field is treated in the optically-thick approximation, with the opacity contributed by Thomson scattering and thermal bremsstrahlung. Our analysis is concentrated on a detailed numerical investigation of hot two-dimensional, Bondi-Hoyle accretion flows with various Mach numbers. We find significant differences with respect to purely hydrodynamical evolutions. In particular, once the system relaxes to a radiation-pressure dominated regime, the accretion rates become about two orders of magnitude smaller than in the purely hydrodynamical case, remaining however super-Eddington as are the luminosities. Furthermore, when increasing the Mach number of the inflowing gas, the accretion rates become smaller because of the smaller cross section of the black hole, but the luminosities increase as a result a stronger emission in the shocked regions. Overall, our approach provides the first self-consistent calculation of the Bondi-Hoyle luminosity, most of which is emitted within r~100 M from the black hole, with typical values L/L_Edd ~ 1-7, and corresponding energy efficiencies eta_BH ~ 0.09-0.5. The possibility of computing luminosities self-consistently has also allowed us to compare with the bremsstrahlung luminosity often used in modelling the electromagnetic counterparts to supermassive black-hole binaries, to find that in the optically-thick regime these more crude estimates are about 20 times larger than our radiation-hydrodynamics results.

astro-ph.HE↗

arXiv2015

Constraints on particle acceleration sites in the Crab Nebula from relativistic MHD simulations

The Crab Nebula is one of the most efficient accelerators in the Galaxy and the only galactic source showing direct evidence of PeV particles. In spite of this, the physical process behind such effective acceleration is still a deep mystery. While particle acceleration, at least at the highest energies, is commonly thought to occur at the pulsar wind termination shock, the properties of the upstream flow are thought to be non-uniform along the shock surface, and important constraints on the mechanism at work come from exact knowledge of where along this surface particles are being accelerated. Here we use axisymmetric relativistic MHD simulations to obtain constraints on the acceleration site(s) of particles of different energies in the Crab Nebula. Various scenarios are considered for the injection of particles responsible for synchrotron radiation in the different frequency bands, radio, optical and X-rays. The resulting emission properties are compared with available data on the multi wavelength time variability of the inner nebula. Our main result is that the X-ray emitting particles are accelerated in the equatorial region of the pulsar wind. Possible implications on the nature of the acceleration mechanism are discussed.

astro-ph.HE↗

arXiv2014

A perturbative approach to the hydrodynamics of heavy ion collisions

Initial fluctuations in hydrodynamic fields such as energy density or flow velocity give access to understanding initial state and equilibration physics as well as thermodynamic and transport properties. We provide evidence that the fluid dynamic propagation of fluctuations of realistic size can be based on a background-fluctuation splitting and a systematic perturbative expansion in the fluctuating fields. Initial conditions are characterized by a Bessel-Fourier expansion for single events, event-by-event correlations and probability distributions. The evolution equations can be solved order-by-order in the expansion which allows to study the fluid dynamical propagation of single modes, the study of interaction effects between modes, the determination of the associated particle spectra and the generalization of the whole program to event-by-event correlations and distributions.

hep-ph↗

arXiv2014

Heavy Ion Collision evolution modeling with ECHO-QGP

We present a numerical code modeling the evolution of the medium formed in relativistic heavy ion collisions, ECHO-QGP. The code solves relativistic hydrodynamics in $(3+1)-$D, with dissipative terms included within the framework of Israel-Stewart theory; it can work both in Minkowskian and in Bjorken coordinates. Initial conditions are provided through an implementation of the Glauber model (both Optical and Monte Carlo), while freezeout and particle generation are based on the Cooper-Frye prescription. The code is validated against several test problems and shows remarkable stability and accuracy with the combination of a conservative (shock-capturing) approach and the high-order methods employed. In particular it beautifully agrees with the semi-analytic solution known as Gubser flow, both in the ideal and in the viscous Israel-Stewart case, up to very large times and without any ad hoc tuning of the algorithm.

hep-ph↗

arXiv2014

Numerical simulations of stellar jets and comparison between synthetic and observed maps: clues to the launch mechanism

High angular resolution spectra obtained with the Hubble Space Telescope Imaging Spectrograph (HST/STIS) provide rich morphological and kinematical information about the stellar jet phenomenon, which allows us to test theoretical models efficiently. In this work, numerical simulations of stellar jets in the propagation region are executed with the PLUTO code, by adopting inflow conditions that arise from former numerical simulations of magnetized outflows, accelerated by the disk-wind mechanism in the launching region. By matching the two regions, information about the magneto-centrifugal accelerating mechanism underlying a given astrophysical object can be extrapolated by comparing synthetic and observed position-velocity diagrams (PVDs). We show that quite different jets, like those from the young T Tauri stars DG-Tau and RW-Aur, may originate from the same disk-wind model for different configurations of the magnetic field at the disk surface. This result supports the idea that all the observed jets may be generated by the same mechanism.

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arXiv2013

How (non-) linear is the hydrodynamics of heavy ion collisions?

We provide evidence from full numerical solutions that the hydrodynamical evolution of initial density fluctuations in heavy ion collisions can be understood order-by-order in a perturbative series in deviations from a smooth and azimuthally symmetric background solution. To leading linear order, modes with different azimuthal wave numbers do not mix. Quadratic and higher order corrections are small and can be understood as overtones with corresponding wave numbers.

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arXiv2013

On the MHD modeling of the Crab Nebula radio emission

In recent years, it has become a well-established paradigm that many aspects of the physics of Pulsar Wind Nebulae (PWNe) can be fully accounted for within a relativistic MHD description. Numerical simulations have proven extremely successful in reproducing the X-ray morphology of the Crab Nebula, down to very fine detail. Radio emission, instead, is currently one of the most obscure aspects of the physics of these objects, and one that holds important information about pulsar properties and their role as antimatter factories. Here we address the question of radio emission morphology and integrated spectrum from the Crab Nebula, by using for the first time an axisymmetric dynamical model with parameters chosen to best reproduce its X-ray morphology. Based on our findings we discuss constraints on the origin of the radio emitting particles.

astro-ph.HE↗

arXiv2010

EM counterparts of recoiling black holes: general relativistic simulations of non-Keplerian discs

We investigate the dynamics of a circumbinary disc that responds to the loss of mass and to the recoil velocity of the black hole produced by the merger of a binary system of supermassive black holes. We perform the first two-dimensional general relativistic hydrodynamics simulations of \textit{extended} non-Keplerian discs and employ a new technique to construct a "shock detector", thus determining the precise location of the shocks produced in the accreting disc by the recoiling black hole. In this way we can study how the properties of the system, such as the spin, mass and recoil velocity of the black hole, affect the mass accretion rate and are imprinted on the electromagnetic emission from these sources. We argue that the estimates of the bremsstrahlung luminosity computed without properly taking into account the radiation transfer yield cooling times that are unrealistically short. At the same time we show, through an approximation based on the relativistic isothermal evolution, that the luminosity produced can reach a peak value above $L \simeq 10^{43} \ {\rm erg/s} $ at about $\sim 30\,{\rm d}$ after the merger of a binary with total mass $M\simeq 10^6 M_\odot$ and persist for several days at values which are a factor of a few smaller. If confirmed by more sophisticated calculations such a signal could indeed lead to an electromagnetic counterpart of the merger of binary black-hole system.

astro-ph.HE↗

arXiv2007

Simulated synchrotron and Inverse Compton emission from Pulsar Wind Nebulae

We present a complete set of diagnostic tools aimed at reproducing synthetic non-thermal (synchrotron and/or Inverse Compton, IC) emissivity, integrated flux energy, polarization and spectral index simulated maps in comparison to observations. The time dependent relativistic magnetohydrodynamic (RMHD) equations are solved with a shock capturing code together with the evolution of the maximum particles energy. Applications to Pulsar Wind Nebulae (PWNe) are shown.

astro-ph↗

arXiv2006

Simulated synchrotron emission from Pulsar Wind Nebulae

A complete set of diagnostic tools aimed at producing synthetic synchrotron emissivity, polarization, and spectral index maps from relativistic MHD simulations is presented. As a first application we consider here the case of the emission from Pulsar Wind Nebulae (PWNe). The proposed method is based on the addition, on top of the basic set of MHD equations, of an extra equation describing the evolution of the maximum energy of the emitting particles. This equation takes into account adiabatic and synchrotron losses along streamlines for the distribution of emitting particles and its formulation is such that it is easily implemented in any numerical scheme for relativistic MHD. Application to the axisymmetric simulations of PWNe, analogous to those described by Del Zanna et al. (2004, A&A, 421, 1063), allows direct comparison between the numerical results and observations of the inner structure of the Crab Nebula, and similar objects, in the optical and X-ray bands. We are able to match most of the observed features typical of PWNe, like the equatorial torus and the polar jets, with velocities in the correct range, as well as finer emission details, like arcs, rings and the bright knot, that turn out to arise mainly from Doppler boosting effects. Spectral properties appear to be well reproduced too: detailed spectral index maps are produced for the first time and show softening towards the PWN outer borders, whereas spectral breaks appear in integrated spectra. The emission details are found to strongly depend on both the average wind magnetization (here approximately 2%), and on the magnetic field shape.

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arXiv2005

Primitive Variable Solvers for Conservative General Relativistic Magnetohydrodynamics

Conservative numerical schemes for general relativistic magnetohydrodynamics (GRMHD) require a method for transforming between ``conserved'' variables such as momentum and energy density and ``primitive'' variables such as rest-mass density, internal energy, and components of the four-velocity. The forward transformation (primitive to conserved) has a closed-form solution, but the inverse transformation (conserved to primitive) requires the solution of a set of five nonlinear equations. Here we discuss the mathematical properties of the inverse transformation and present six numerical methods for performing the inversion. The first method solves the full set of five nonlinear equations directly using a Newton-Raphson scheme and a guess from the previous timestep. The other methods reduce the five nonlinear equations to either one or two nonlinear equations that are solved numerically. Comparisons between the methods are made using a survey over phase space, a two-dimensional explosion problem, and a general relativistic MHD accretion disk simulation. The run-time of the methods is also examined. Code implementing the schemes is available for download on the web.

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arXiv2004

Transverse oscillations in solar coronal loops induced by propagating Alfvenic pulses

The propagation and the evolution of Alfvenic pulses in the solar coronal arcades is investigated by means of MHD numerical simulations. Significant transverse oscillations in coronal loops, triggered by nearby flare events, are often measured in EUV lines and are generally interpreted as standing kink modes. However, the damping times of these oscillations are typically very short (from one to a few periods) and the physical mechanism responsible for the decay is still a matter of debate. Moreover, the majority of the observed cases actually appears to be better modeled by propagating, rather than standing, modes. Here we perform 2.5-D compressible MHD simulations of impulsively generated Alfven waves propagating in a potential magnetic arcade (assumed as a simplified 2-D loop model), taking into account the stratification of the solar atmosphere with height from the photosphere to the corona. The results show a strong spreading of the initially localized pulses along the loop, due to the variations in the Alfven velocity with height, and correspondingly an efficient damping of the amplitude of the oscillations. We believe that simple explanations based on the effects of wave propagation in highly inhomogeneous media may apply to the majority of the reported cases, and that variations of the background density and Alfven speed along the loop should be considered as key ingredients in future models.

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