GR-PLUTO · General Relativistic numerical models of accretion disks and magnetic reconnection with the PLUTO code

General Relativistic numerical models of accretion disks and magnetic reconnection with the PLUTO code

In the last few years, the scientific research on astrophysical black holes (BHs) has produced some historical discoveries, such as the detection of gravitational waves and the imaging of supermassive black holes with their accretion disk at the center of galaxies, including our own Milky Way. One of the main ingredients that made these achievements possible is the theoretical understanding of how magnetic fields shape the dynamics of the gas around black holes. Matter in such extreme gravitational fields assumes the form of hot plasma, i.e. a state where charged particles such protons and electrons are not bound within atoms and travel much more freely under the influence of electromagnetic fields. A primary example of physical process that can take place in plasmas is magnetic reconnection, i.e. the rearrangement of the magnetic field's own structure, which converts magnetic energy into kinetic and thermal energy. One of the main consequences of reconnection is the acceleration of particles to velocities close to the speed of light, which then produces high-energy non-thermal radiation that we can observe from accreting black holes. Since astrophysical plasmas are generally non-collisional, the coupling of particles and magnetic fields leads to kinetic instabilities and dissipation of magnetic energy at spatial scales much smaller than the size of a black hole. Particle-In-Cell (PIC) simulations are a chief instrument for the investigation of plasma dynamics at small scales, but they are generally too computationally expensive to model black holes and their accretion disks. General relativistic magnetohydrodynamic (GRMHD), on the other hand, offers a more suitable framework to describe the behavior of accretion disks around black holes, doing so by treating particles and electromagnetic fields as a single magnetized fluid. GRMHD models can be extended to include magnetic dissipation, but they generally consider a constant electric conductivity, which generally lead to results that are not in agreement with first principles PIC models. The goal of the GR-PLUTO project is to perform the first numerical study on reconnection in relativistic disks using an effective non-constant resistivity, which will go beyond standard fluid models by introducing a new non-collisional effect in the way that magnetic fields are dissipated. This approach will test our current knowledge of relativistic magnetic reconnection and provide more consistent estimates for the rate at which particles can be accelerated and the general structure of the magnetized accretion flow around a black hole. Since the required simulations will have a high computational cost, a significant part of the project is devoted to the development a numerical tool for the modeling of GRMHD flows that combines the benefits of highly accurate numerical schemes with energy-efficient computational methods. To this end the freely-distributed astrophysical code PLUTO is extended (in collaboration with the PLUTO development team at the physics department of the University of Turin) to include the effects of General Relativity, complementing it with high-order integration schemes and GPU-accelerated routines.

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