Rapid accretion and state changes in strongly magnetized disks

Accretion disks power many of the universe’s most luminous phenomena, acting as intermediaries that enable matter to shed angular momentum and accrete onto central objects like black holes or stars. While angular momentum transport in disks, often driven by turbulence from the magnetorotational instability (MRI), has been extensively studied, significant challenges remain in reconciling simulations with observed accretion rates and understanding state transitions in systems such as X-ray binaries and quasars.

In this talk, I explore how strongly magnetized 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 simulations, in which such a disk forms self-consistently around a black hole and supports super-Eddington accretion rates [Hopkins, Squire et al. OJA (2024)]. Using local shearing-box simulations, 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 sufficiently strong azimuthal fields are present, allowing the system to sustain a Parker-instability-driven dynamo. While many aspects of this behavior remain uncertain, it offers a promising pathway to reconcile simulations and observations, with interesting implications for quasars and other rapidly accreting systems.