My current research interests focus on the development and application of numerical schemes for General Relativistic MHD (GRMHD).

Part of my work focuses on the study of outflows from compact objects, their interaction with the environment, and in particular to pulsar winds and Pulsar Wind Nebulae (PWNe). Particular efforts have been devoted to the study of Crab-like or plerionic Supernova Remnants (SNR), for what concerns both models of specific objects and evolution of relativistic plasma. Such research aims at increasing our understanding of the physical processes in relativistic winds, and will have further implication for relativistic outflows more generally.

Another field where I am involved is the development of numerical model of magnetized compact objects in GR and beyond. I have developed a numerical code XNS that solves the coupled Maxwell-Euler-Einstein equations, for rotating magnetized Neutron Stars. And also a time dependent GR-MHD code X-ECHO, for the study of collapsing/rotating systems. I am also involved in the development of GR-MHD schemes to non-ideal and dynamo/chiral effects.

In the past I have worked on the so called "millisecond magnetar" model for long duration Gamma Ray Bursts (GRBs). We have developed a series a numerical simulation of the interaction of neutrino driven outflow from the newly born magnetar, with the surrounding progenitor, showing for the first time the self consistent formation of a jet, and its propagation and acceleration trough the star.

The ECHO Code

ECHO is an high order eulerian conservative scheme, that was developed in collaboration with Luca del Zanna, Pasquale Londrillo, and Olindo Zanotti for the study of Special Relativistic MHD and General Relativistic MHD on fixed metric (like Kerr metric for accretion onto a black Hole). The code uses a central type scheme (HLL) in order to avoid the complexity of the full characteristic decomposition. This increase the flexibility of the code (not limited by the knowledge of the eigenstructure of the equations) and the use of high order allows one to reduce the numerical diffusion associated with the central scheme. We are able to achieve Lorentz factor up to ~100. The code properly treats the magnetic monopoles constraint, which is enforced to round off machine error, by using the constrained transport method. The code is currently maintained by Luca Del Zanna.

The X-ECHO Code

X-ECHO is a code for axisymmetric General Relativistic MHD, with dynamical space-time, that I am currently developing together with Luca Del Zanna. It allows the use of Ideal, Resistive, and Dymnamo/Chiral closures for the currents. The code combines the fluid HD/MHD module of ECHO to a new metric solver, that I have written. The metric solver belongs to the class of the Fully Constrained Schemes (FCF) where Einstein equations are solved as a set of elliptic equations. In particular we used the so called Extended Conformally Flat Approximation (XCFC). This allow us to solve the metric equations in a hierarchical way, decoupling them from solution of the fluid variables, and ensuring stability. The code has been widely tested, even in strongly dynamical systems evolving to BH.

Neutron Stars

Neutron Stars (NSs) are the most compact objects in the universe endowed with an internal structure. They are one of the most fascinating lab for extreme physics. The code I developed, XNS, solves for the axisymmetric equilibrium configuration of differentially rotating Neutron Stars with toroidal, poloidal and mixed magnetic field configurations, using the XCFC approximation for the metric, in spherical coordinates. The code is based on the metric module and routines developed for the GR-MHD code X-ECHO. We have applied it to the study of magnetized NSs, quark phase-transition in rotating NSs, Dynamo, and more recently for the first time to NSs in Scalar Tensor Theories.

Pulsar Wind Nebulae

I began the study of PWNe applying our RMHD code to the study of the evolution (1D spherically symmetric) of PWNe inside SNR, to verify the accuracy of previous simplified analytic solution. More important results came from multidimensional studies, showing that if the energy flux in the pulsar wind is higher at the equator than at the pole (as in the split monopole model), magnetic hoop stresses in the post shock region can divert part of the equatorial flow toward the axis, collimating and accelerating it, which explains the often jet-torus morphology observed at high energy. Our model have been further developed to include emission from radio to X-ray to TeV. By building synchrotron maps based on our numerical results we were able to recover the main observed features such as the inner ring, the torus, the jet and the inner knot. We also found that the best agreement between observations and synthetic maps requires a wind with magnetic field vanishing at the equator (as in the striped wind model).

Bow Shock PWNe

More recently I have worked on multidimensional models of the interaction of a pulsar wind with the ambient medium in case of pulsar moving through the ISM. This is an extension of my previous work on Bow-Shock PWNe. I have investigated how the dynamics depends on the magnetization of the wind, the spin-axis inclination, the strength and direction of the ISM magnetic field. We have also computed emission maps for the non-thermal component, and investigated the confinement of high energy particles.

Magnetar GRBs

My research has largely focused on showing that newly formed magnetars, if rotating rapidly, will naturally produce collimated relativistic outflows as the neutron star cools in the first ~ 100 sec of its life. We have computed a detailed model for the evolution of a millisecond-magnetar and shown that the outflow parameters match the one seen both in long and short GRBs. I have shown that collimated outflow can result from the interaction.

Osservatorio Astrofisico di Arcetri. L.go Fermi 5, 50125 Firenze Italy

Tel: +39 055 2752285, Fax: +39 055 2752