PhD defense by Filippo Camilloni

Analytical Approaches to Relativistic Astrophysics

We exploit analytical methods of General Relativity and techniques of relativistic hydrodynamics to address novel theoretical aspects of relativistic astrophysics. One of our main purpose is the study of astrophysical spinning black holes, for instance active galactic nuclei, and the magnetospheres that surround them, which are supposed to be responsible for the emission of relativistic jets of particles that are launched near the poles. The primary candidate for the mechanism behind this emission is an electromagnetic manifestation of the Penrose process called Blandford & Znajek mechanism, which has been mostly studied numerically and poorly understood from an analytical perspective.

We address this by considering a relativistic theory which captures the dynamics of the coupling between plasmas and magnetic fields, namely MagnetoHydroDynamics (MHD), and more specifically its low-density regime Force-Free Electrodynamics (FFE). We consider magnetospheres around Kerr black holes which are either in the slow spinning regime, at extremality and near-extremality. The first regime requires the enhancing the Blandford & Znajek perturbation theory with a matched asymptotic expansion scheme, which reveals the ineluctability of non-analytic terms in the energy and angular momentum extraction rates. Besides improving considerably the agreement between theoretical models and numerical simulations in this regime, the new non-analytic terms that we discovered potentially shed light over the non-perturbative structure of spinning black hole magnetospheres.
For black holes in the extreme and near-extreme case, the prominent role of two force-free attractors in the near-horizon geometries are exploited to develop a novel perturbation theory which allows us to attain magnetically-dominated magnetospheres.

Finally, we focus on extending relativistic MHD outside the ideal order by incorporating dissipative effects in the theory. This can be consistently by exploiting a recent dual formulations which exploits the prominent role of 1-form global symmetries in electromagnetism coupled to matter. The inclusion of transport coefficients in relativistic theories is often associated to the appearence of instabilities of equilibrium states and modes propagating superluminally. We take care of causality and stability issues for the case of MHD by exploiting a new approach recently developed in the framework of relativistic hydrodynamics. This is a first stable and causal model of relativistic dissipative MHD which constitutes a promising alternative to Müller-Israel-Stewart-type theories to study dissipations in various context ranging from heavy-ions collisions to astrophysics.

Committee members: Niels Obers, Maria J. Rodriguez and Geoffrey Compère
Supervisors: Troels Harmark and Gianluca Grignani