A&A Colloquium: Matthew Liska (Harvard University)

3:30–4:30 pm ERC 161

Host: Hsiao-Wen Chen

Matthew Liska (Harvard University)


How do the most luminous black holes accrete and expel gas?


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.  To date, our theoretical knowledge of such `luminous’ black hole accretion disks mostly relies on semi-analytical models, supplemented by a very limited set of numerical simulations. I will discuss results from the first radiative high resolution GRMHD simulations of a L=0.35 Ledd accretion disks. I will demonstrate the importance of radiative cooling on the disk structure and discuss how magnetic fields can potentially lead to the formation of a hot corona: A billion degree plasma whose origin is unknown. In the second part of my talk I will drop a crucial assumption that most analytical models and simulations make: Perfect alignment between the disk and black hole spin axes. I will demonstrate that frame-dragging by the black hole can tear a misaligned luminous accretion disk apart and that accretion proceeds through shocks. This challenges the 30 year old paradigm that invokes magnetized turbulence to explain the accretion of gas. I will present synthetic lightcurves of misaligned accretion disks where a combination of the tearing process, disk precession and radial-epicyclic oscillations leads to quasi-periodic variability compatible with X-Ray binaries and active galactic nuclei. I will finish my talk by briefly touching on the challenges associated with modelling similar physics in the context of other transients including neutron star binary mergers and core-collapse supernovae.

About speaker:

Matthew Liska is a computational astrophysicist with an interdisciplinary interest in astrophysics, plasma physics and computation. He is the main developer of the world's first GPU accelerated GRMHD code ("H-AMR") which he uses to study especially challenging problems in black hole accretion and plasma physics. These include (tilted) accretion disks, large scale jets, tidal disruption events, binary mergers and the generation of large scale magnetic fields.  As a John Harvard fellow he is currently leading the development of a new radiative transfer module for H-AMR, which will be used to study black hole state transitions and binary merger remnant disks at record breaking resolutions and for unprecedented runtimes.

Event Type


Jan 11