We present a “cyclic zoom” method to capture the dynamics of accretion
flows onto black holes across a vast range of spatial and temporal scales in
general relativistic magnetohydrodynamic (GRMHD) simulations. In this method,
we cyclically zoom out (derefine) and zoom in (refine) the simulation domain
while using a central mask region containing a careful treatment of the
coarsened fluid variables to preserve the small-scale physics, particularly the
magnetic field dynamics. The method can accelerate GRMHD simulations by
$\gtrsim 10^5$ times for problems with large scale separation. We demonstrate
the validity of the technique using a series of tests, including spherically
symmetric Bondi accretion, the Blandford-Znajek monopole, magnetized turbulent
Bondi accretion, accretion of a magnetized rotating torus, and the long-term
evolution of an accreting torus about both Schwarzschild and Kerr black holes.
As applications, we simulate Bondi and rotating torus accretion onto black
holes from galactic scales, covering an extremely large dynamic range. In Bondi
accretion, the accretion rate is suppressed relative to the Bondi rate by
$\sim(10r_\mathrm{g}/r_\mathrm{B})^{1/2}$ with a feedback power of $\sim 0.01
\dot{M} c^2$ for vanishing spin, and $\sim 0.1 \dot{M} c^2$ for spin
$a\approx0.9$. In the long-term evolution of a rotating torus, the accretion
rate decreases with time as $\dot{M}\propto t^{-2}$ on timescales much longer
than the viscous timescale, demonstrating that our method can capture not only
quasi-steady problems but also secular evolution. Our new method likewise holds
significant promise for applications to many other problems that need to cover
vast spatial and temporal scales.

Cet article explore les excursions dans le temps et leurs implications.

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