Monday, January 31, 2022 at 3:35pm

JILA Auditorium and https://cuboulder.zoom.us/j/96468671083

Matthew Liska, Harvard-Smithsonian Center for Astrophysics

"Formation of a Hot Corona and Quasi-periodic Oscillations in General Relativistic MHD Simulations of Luminous Black Hole Accretion Disks"

Abstract:

The gravitational pull of a black hole attracts gas and forms a physical laboratory whose extreme conditions cannot be replicated on Earth. The infalling gas forms an accretion disk where the interplay between hydromagnetic processes and the warping of space-time releases gravitational energy in the form of radiation, relativistic jets, and winds. It is likely that most gas falls into black holes when the accretion rate approaches the Eddington limit, at which point radiation pressure overcomes gravity. Observations suggest that such luminous black holes can accrete in various spectral states, some of which feature a 'hot' corona alongside an accretion disk and powerful jets. While the corona can make up most of the detected emission, we do not know how a corona forms from the accretion disk or what it looks like. In the first part of this talk I will present the first radiative two-temperature general relativistic magnetohydrodynamics (GRMHD) simulations of luminous accretion disks, which were enabled by running my GPU-accelerated GRMHD code 'H-AMR' on 6000 GPUs of OLCF Summit, the nation's largest supercomputer. I will demonstrate that large scale vertical magnetic fields threading the accretion disk automatically lead to the formation of a hot corona alongside powerful jets. I will discuss the role of magnetic reconnection in equatorial current sheets in heating up the coronal plasma to temperatures exceeding 5x10^8K for electrons and 10^10K for ions. In the second part of my talk, I will demonstrate that when luminous accretion disks are misaligned with the black hole spin axis, the warping of space- time by the spinning black hole can tear such disks apart. This leads to disk precession and changes the structure and dynamics of accretion disks to the point that contemporary theoretical models lose their validity (see my figure). Using general relativistic ray-tracing I will demonstrate that disk tearing can potentially explain multiple types of quasi-periodic oscillations (QPOs) in the lightcurves of black holes, which have remained a mystery for many decades.

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