Current PhD Thesis Fellows

#Virginia Bresci

I was born in Florence, Italy, where I pursued my studies in general Physics before, for my Bachelor, and in Astrophysics, for my Master, later.

My Master Thesis focused on Cosmic Rays propagation in the Galaxy. The ratio between secondary and primary cosmic rays (CRs) is the most important observable source of information we have. Primary cosmic rays are thought to be accelerated mainly in Supernova Remnant shocks and then released in the interstellar medium where secondary particles can be created by occasional collisions with interstellar matter. As a result, the ratio between the fluxes of secondary and primary particles carries information about the amount of matter CRs have encountered and is expected to monotonically decrease with energy roughly as the inverse of the diffusion coefficient. Recent measurements by AMS-02 revealed some deviations from this behavior that might be explained by taking into account two effects: firstly, some fraction of secondary particles can be produced within the acceleration region, and hence undergo acceleration immediately after birth, and secondly, there is a non-negligible probability that secondary particles encounter an accelerator (and are reaccelerated) during propagation. Modelling and accounting for both effects allowed us to well-reproduce both the fluxes of the best measured nuclei and the secondary-to-primary ratios mentioned above.

In October 2019, I started a doctoral thesis at the Institut d’Astrophysique de Paris under the supervision of Martin Lemoine (IAP) and Laurent Gremillet (CEA, Bruyères-le-Chatel), partly funded by Institut Lagrange de Paris.

The main project of my thesis aims to model in part the complex nebular region surrounding young pulsars, where extreme plasma conditions (non-reproducible in laboratories) are achieved. The Pulsar’s Nebulae is filled by an ultra-relativistic non-collisional flow of turbulent highly magnetised pairs of electrons and positrons. Near the central object, the flow of particles is believed to be ultra relativistic and strongly magnetised so that the energy is mainly carried in the form of Poynting flux, but approaching the nebula the flow becomes slow and weakly magnetised. The particles passively follow the flow in the internal region but have a considerable energy in the outer region. How the magnetic energy has been dissipated into disordered energies of particles between these two zones is one of the most important questions of the domain. A deep insight into these phenomena requires to refine at the kinetic level the MHD description of the nebula, which successfully models its behaviour and morphology but at the cost of theoretical assumptions which concern the efficiency of the dissipation of magnetic energy and the acceleration of particles.

My thesis work thus combines both analytical models and numerical studies on simulations reproducing, among all, this nebular environment using a massively parallel particle-in-cell code (Calder) provided by CEA, in order to understand in details how the interaction between turbulence and shock affects the shock dissipation process, the structure of the collisionless shock itself, and the particle acceleration process. Even if our project deals with the theme of pulsar nebulae, this problem of the interaction between shock and turbulence is general and has never been tackled in the relativistic limit.

Contact: virginia.bresci[at]