GRMHD simulations of thick accretion disks in the Event Horizon Telescope era: the role of the mean-field dynamo mechanism

The baryonic matter of the Universe is found almost entirely in the form of plasma, or ionized gas where the moving charges interact with each other through the self-generated currents and the related magnetic fields within a highly material conductive. The energy contained in the plasma in terms of the magnetic field is usually comparable to the kinetic and/or thermal energy of the plasma itself, and many violent observed phenomena are attributable to the sudden release of magnetic energy (e.g. flares and coronal mass ejection in the Sun, gamma ray bursts, gamma flare in magnetars, relativistic jets in active galactic nuclei). Since the Universe may not have formed magnetized, a natural question one needs to answer is what processes can lead to a weak magnetic field, from zero initial fields. Battery-like mechanisms are needed to create primordial extra- galactic fields, which may be amplified to higher values by plasma advection, rotation and collapse to values appropriate for stellar magnetism, up to B ∼ 1012 G, the field of a standard neutron star, a value required to power the surrounding young supernova remnant. Most of these batteries lead to field strengths much weaker than the observed field. So some way of amplifying the field is required. Magnetic fields of small scale and large scale components are observed in various astrophysical settings. This thesis work is focused on the investigation of the amplification mechanisms of magnetic fields in system of gas rotating around a supermassive black hole, following the renewed interest given by the recent imaging of the M87 black hole at millimetre wavelengths by the Event Horizon Telescope (EHT). In most cases an amplification of the magnetic fields may occur also by instabilities capable of converting kinetic energy into magnetic energy. A very efficient and ideal process is the magneto-rotational instability (MRI) that provided a local mechanism, effcient for a wide range of magnetic field strength, which leads to a growth on dynamical time-scales of linear perturbations and naturally develops MHD turbulence. The only necessary condition for its onset is the presence of a differentially rotating fluid threaded by a weak magnetic field. However, the amplification of the magnetic field is a non-ideal process due to the non-linear coupling of small-scale velocity and magnetic field fluctuations, possibly caused by the MRI. The result of this correlation leads to the creation of an electromotive force capable of amplifying magnetic fields. This process is known as mean-field dynamo and has been applied to a large number of astrophysical contexts. Currently, GRMHD simulations of MRI-induced accretion on to rotating black holes are being receiving considerable attention due to the success of the EHT collaboration, capable of imaging the emission and the shadow around the event horizon of a black hole for the very first time. The aim of this thesis work is to provide an alternative numerical accretion modelling to the ideal one in which the initial magnetic field has a well-defined poloidal structure and an intensity not exactly negligible. The mean-field dynamo allows us to investigate the possibility of producing poloidal field necessary for the development of MRI and the launch of jets even starting from the most unfavorable condition, that is an initial toroidal field with extremely lower magnetization than those used in ideal GRMHD simulations. In this work we have investigated, for the first time by means of non-ideal axisymmetric GRMHD simulations, the mean-field dynamo process operating in thick accretion disks around black holes, in the fully non-linear regime. Combined with the differential rotation of the disk, the dynamo process is able to produce an exponential growth of any initial seed magnetic field up to the values required to explain the observations,when the instability tends to saturate even in the absence of artificial quenching effects. Before reaching the final saturation stage we observe a secondary regime of exponential growing, where the magnetic field increases more slowly due to accretion, which is modifying the underlying equilibrium. In the stationary state characterized by the saturation of the magnetic field growth, the dynamo is able to remove the angular momentum and trigger the accretion. Finally, we show that it is possible to reproduce the main diagnostics present in the literature by starting from very unfavorable initial configurations, such as a purely toroidal magnetic field with negligible magnetization. In parallel, we present the contribution to the code Comparison Project that aims to compare ideal GRMHD solutions for the evolution of a magnetized accretion flow in two distinct regimes where turbulence is promoted by the magnetorotational instability.