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SUMMARY:Rapid accretion and state changes in strongly magnetised disks - J
 onathan Squire [Otago\, New Zealand]
DTSTART:20250605T110000Z
DTEND:20250605T120000Z
UID:TALK231373@talks.cam.ac.uk
CONTACT:Loren E. Held
DESCRIPTION:Accretion disks power many of the universe’s most luminous p
 henomena\, acting as intermediaries that enable matter to shed angular mom
 entum and accrete onto stars or compact objects. While angular momentum tr
 ansport in disks has been extensively studied\, especially in the context 
 of magneto-rotational turbulence\, significant challenges remain. These in
 clude reconciling simulation results with observed accretion rates and und
 erstanding state transitions in cataclysmic variables\, x-ray binaries\, a
 nd quasars. \n\n\nIn this talk\, I explore how strongly magnetised disks 
 — where azimuthal magnetic fields dominate\, with energies exceeding the
  plasma’s thermal energy — may help resolve these issues. Interest in 
 this regime is motivated by recent “hyper-refined” cosmological simula
 tions\, in which such a disk forms self-consistently around a black hole a
 nd supports super-Eddington accretion rates. Using local shearing-box simu
 lations\, we identify two distinct turbulent states: the previously known 
 "high-β" state with modest accretion stresses (α << 1) and weak magnetic
  fields\, and a new "low-β" state with strong\, self-sustaining azimuthal
  magnetic fields\, supersonic turbulence\, and rapid accretion (α ≈ 1).
  The transition between these states is abrupt and occurs when sufficientl
 y strong azimuthal fields are present\, allowing the system to sustain a P
 arker-instability-driven dynamo. While many aspects of this behaviour rema
 in uncertain\, it offers a promising pathway to reconcile simulations and 
 observations\, with interesting implications for quasars and other rapidly
  accreting systems.
LOCATION:MR12 DAMTP and online
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