CRA Seminar: Nick Kaaz – Northwestern UN
September 12 @ 3:30 pm – 4:30 pm
Title: Challenging the standard accretion disk paradigm with warps, strong magnetic fields and super-Eddington accretion in quasar disks
Abstract: “Classical” accretion disks are geometrically thin, radiatively efficient and mechanized by turbulent viscosity. Yet, many observational and theoretical issues challenge this paradigm, especially in quasars. This is perhaps unsurprising, as classical disks do not take into account certain key physics. For instance, the infalling gas that eventually forms a disk around a black hole has no prior knowledge of the black hole spin axis. Thus, most accretion disks will be at least initially misaligned with respect to the black hole spin axis. As the black hole rotates, it drags the surrounding space-time, inducing ‘Lense-Thirring’ torques which cause the disk to undergo precession and become warped. This can drastically alter the accretion process and drive rapid variability, similar to that which is seen in changing-state active galactic nuclei. Additionally, quasars are generally fed from cold, highly magnetized gas complexes, which determines the initial conditions of the disk. This can result in a disk that is magnetically dominated, enabling high accretion rates that are sometimes above the Eddington limit, which has important consequences for the inner accretion flow. In this talk, I will discuss my recent work on both radiative and non-radiative general-relativistic magnetohydrodynamic simulations of both thin, highly tilted accretion disks and super-Eddington, highly magnetized disks around rapidly rotating black holes. In the former case, I will present novel dissipation mechanisms unique to these systems that drive accretion on timescales much shorter than the usual viscous time. In the latter case, I will discuss the physics that governs the magnetic state of the disk, the resulting emission and outflow properties, and the implications for the cosmological growth of supermassive black holes.
Event info here.
