SARIs : a new paradigm for turbulent accretion onto black holes

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• Rony Keppens (KU Leuven, Belgium)
• Monday 04 November 2024, 14:00-15:00
• MR14 DAMTP and online.

If you have a question about this talk, please contact Loren E. Held.

During the last three decades, the astrophysical literature collected ever growing evidence that magnetic fields, however weak, play a decisive role in triggering turbulent flow in accretion disks. A process referred to by its acronym MRI , for magneto-rotational instability, provides a universally accepted explanation of why magnetized accretion disks turn turbulent at all. Magnetic fields and a rotation rate that decreases away from the central object, introduce a runaway process governed by conservation of angular momentum.

However [1], the MRI diverts our attention to special (axisymmetric, i.e. with no variation about the disk center) natural eigenfrequencies of the disk. This focus on eigenfrequencies is physically sound, just like we appreciate music through the natural vibrations of a guitar string. Curiously enough, a rigorous mathematical analysis now reveals that we may as well go beyond these pure natural resonances, and look for any growing wave package that just needs a tiny amount of added energy (just like the guitar player does by plucking the string). This opens up an entire new window on what drives turbulence in accretion disks, with a decisive role for very localized wave packages that surf the disk at speeds larger than the Nobel-prize winning Alfvén speed. These Super-Alfvénic rotational instabilities – SAR Is for short – are completely unprecedented, and since they no longer adopt artificial axisymmetry, may drive processes enhancing the magnetic field in-situ, acting as a dynamo. This new concept for turbulence in accretion disks truly represents a paradigm-shift, but required the solution of intricate mathematical equations, where singular behavior is key [1,2]. Since singular behavior of mathematical equations gave birth to black holes, it is satisfying to discover that turbulent processes near black holes thrive on singularities as well.

[1] Hans Goedbloed & R. Keppens, 2022, ApJ Supplement Series 259, 65 (41pp), Full paper, doi:10.3847/1538-4365/ac573c [2] N. Brughmans, R. Keppens & H. Goedbloed, 2024, ApJ 968, 19 (20pp), Full paper, doi:10.3847/1538-4357/ad3d52

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