Research Interests

My current research interests focus on the application of recently developed numerical schemes for Relativistic MHD (RMHD) to the study of outflows from compact objects and their interaction with the environment, and in particular to pulsar winds and Pulsar Wind Nebulae (PWNe). Part of my efforts has 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. My recent work has focused 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 consisen formation of a jet, and its propagation and acceleration trough the star. For the future I plan to address the more general problem of the formation of relativistic winds from  neutron stars or accretion disks and their interaction  with the environment, either the ISM or a surrounding Supernova Remnant (SNR), or a GRB progenitor. Such research aims at increasing our understanding of the physical processes in relativistic winds, and will have further implication  for relativistic outflows more generally.

ECHO code

ECHO is an high order eulerian conservative scheme, that was developed in collaboartion with Luca del Zanna, Pasquale Londrillo, for the study of relativistic MHD, and has recently been extended to the caseof
fixed non diagonal metric (like Kerr metric for acretion onto a black Hole). The code was developed using a central type scheme (HLL) in order to avoid the complexity of the full characteristic decomposition, required in wave based solver. This increase the felxibility of the code (not limited by the knowledge of the eigenstructure of the equations) and the use of high order alow to reduce the numerical diffusion associated with the central scheme. Indeed this strategy has proved very successful and had been followed by other groups.

  Most of the astrophysical sources of high-energy radiation, as AGNs, GRBs, microquasars, and PWNe, are believed to involve the presence of relativistic motions in a magnetized plasma.  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.  A future development will be the application of adaptive mesh refinement,  the implementation of different equations of state, and the coupling with the Einstein equations, which is essential to address collapse and merging of compact objects, and the related emission of gravitational waves. Alternatively Conformally Flat Metric could be used for axisymmetric collapse.

We hope to be able to present as soon as possible a public version of the code for external users who might be interesting in doing Relativistic MHD. The code is currently maintained by Luca Del Zanna.

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. 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 (FC
F) where Einstein equations are solved as a set of elliptic equations. In particular we used the so called Extended Conformally Flat Approximation (XCFC). The code is now under testing phase, and we hope to be able to present it soon. In the mean time here is a simple result: the plot shows a Fourier Fransform (a spectrum) of the central density of a stable Neutron Star (the solution of the Tollman-Oppenheimer-Volkoff equations). Due to round-off errors, normal modes oscillations are excited. The plot compares the spectrum of these oscillations, that we find in our simulation, with the known frequencies of the normal modes.

The Crab nebula and Plerions like it

PWNe are bubbles of relativistic plasma arising from the confinement of a pulsar's wind by the surrounding medium (usually the SNR), that shine in non thermal emission from radio to X-ray. For the first time we apply an RMHD code to the study of the evolution (1D spherically symmetric) of PWNe inside SNR, to verify the accuracy of previous simplified analytic solution. However more important results came from multidimensional studies. Recent high resolution observations with Chandra and Hubble have in fact shown that many such objects are characterized by an evident axisymmetric feature know as ``jet-torus structure''. A main torus is often observed with a brighter inner ring, and jets comin
g from the vicinity of the pulsar. Such structure can not be explained in the 1D radial model,  expecially the jet, given the difficulties in  self-collimation of ultra relativistic flows.

Our results show 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. The anisotropic energy flux is also important in shaping the wind termination shock and producing high velocity flow channels in the post shock region. We find that the formation of the jet requires an higher wind magnetization than what previous 1D models assumed. Moreover the shape of the termination shock and the flow pattern in the downstream region lead to very characteristic signature in the high energy emission. 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).

The work on PWNe modeling, is still proceeding, we are now trying to sample more in detail the parameter space in terms of wind latitudinal dependence to derive some criteria in order to correlate the observed structure, their morphology and variability to the conditions in the wind, which in turn can be used to derive some insight into pulsar magnetospheres. Recently we have presented new models of  the spectral and polarization properties of such nebulae. We have also investigated the Gamma-ray comptonized emission, and shown that previous estimate based on very simplified models for the nebular evolution and internal structure, could lead to discrepancy of a factor 5 to 10. Very recently in collaboartion with Serguei Komissarov, we have investigated the time variability, and shown that MHD processes can reproduce with great accuracy also the short term temporal properties of these nebulae.

The magnetar model for GRBs

My work has focused on the  study of neutron star winds, both in the context of pulsars interacting with the ISM, and for proto-neutron stars inside a progenitor star, and in particular I am interested in high energy phenomena during the collapse.  My results to date have clarified the transition between mass dominated and magnetically dominated winds. Based on my previous work on the interaction of pulsar wind with the surrounding SNR, I am at the moment investigating the interaction of proto-magnetar winds with the surrounding progenitor star, as a possible engine for GRBs, and have shown for the first time that relativistic jets can form at large distance even in the case of isotropic outflows emerging from the central engine.

It has been recognized in recent years that magnetic field can play a key role in the core collapse and during the supernova explosion. Numerical techniques and computational facilities have been developed to a point where it is possible to investigate such multidimensional phenomena. In the case of GRBs in particular there is consensus on the important role of magnetic field in the acceleration of outflows but several issues regarding the interaction of the central engine with the surrounding progenitor star still remain.

Two plausible central engines for long-duration GRBs have been identified, either an accretion disk around a black hole (the Collapsar model), or a rapidly rotating magnetar.   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. There are strong indications suggesting that rapidly rotating neutron stars with a strong magnetic field can form during core collapse, and that relativistic outflows will naturally be produced by these objects as they cool down in the first few seconds.  The key question is to understand how these outflows interact with the surrounding stellar progenitor,  under which conditions a collimated jet with the correct lorentz factor is formed, and how the spin-down properties of the neutron star change. Interestingly the problem presents many analogies with much larger system such as Pulsar Wind Nebulae, where we have direct observation of the interaction of the pulsar wind with the SNR.

The association of GRBs with SN clearly show that a correct understanding of one cannot come without a proper understanding of the other. Research on GRB engines have thus important implications in the more general context of core collapse, supernovae and the birth of compact objects. The role played by a possible central source in triggering the explosion, and the effects of  magnetic field (either amplified in the collapse of produced by dynamos) have started to be tackled only recently. Moreover it is important to understand how the outflows produced by the central engine interacts with the surrounding medium, what feedback is expected, and the differences in observational signatures. Obviously results and techniques developed for the case of neutron-star winds have a much broader applicability for the general problem of relativistic outflows from compact sources.

Recent results about the SN-GRB connection have clearly shown that GRBs might provide important informations on the very dynamics involved in the collapse, and short of GW they might be our only way to investigate the formation of compact remnants as BH or NS. The evolution of these systems involves a large dynamic range both in space (from the compact object to the outer stellar layers) and time (from milliseconds to seconds), and a complexity microphysics (radiation pressure, neutrino heating and cooling, nuclear processes). Only recently numerical codes and computational facilities have reached the ability to manage these problems.  My future projects focus on the general problem of core collapse, the nucleosynthesis and its signatures associated with either the GRB jet or more generally to energetic outflows from compact central remnants,  the eventual signature of asymmetric explosion of the SN spectra,  and the formations of compact remnants, and their relation with the progenitor.

Public Outreach

From 2000 to 2004 I have been the coordinator of the local amatorial astronomy club in the city of Agliana (Pistoia, Italy). During this period I have organized and given public lectures, public observations of the night sky and the Sun, and astronomy exhibits. I have been in the organizing committee for the local activities of the Province of Pistoia for the National Week of Scientific Culture (Settimana Nazionale della Cultura Scientifica). I have taught short astronomy classes (4-10 hours), and organized laboratory activities to elementary, middle and high school since 1999. I have done public observations at the Astrophysical Observatory in Arcetri, and I have experience in managing small size telescopes (8-14 inches) and small planetariums.