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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Recordings available at: https://www.youtube.com/watch?v=BIFu3mbxqiw&list=PLupSU3PE5is1fkio36JA9DaYHyTJ9X6Jt