Variability in Black Hole Accretion: Dependence on Rotational and Magnetic Energy Balance

Abstract Most general-relativistic magnetohydrodynamic simulations of black hole (BH) hot accretion flows are initialized with small rotating tori and produce stable jets with only small fluctuations. However, recent studies using larger-scale Bondi-like initial conditions have reported intermittent jet activity and loss of coherent rotation. To investigate these differences, we modify the standard torus setup across four BH spins: a * = 0, 0.5, 0.9, −0.9. First, we increase the torus size significantly (pressure maximum at 500 gravitational radii), allowing long simulations (2.8 × 10 5 gravitational times) without gas depletion. These runs reproduce the weak variability seen in smaller tori, indicating that a larger dynamic range alone does not cause strong fluctuations. We observe moderate suppression of the accretion rate by factors of ∼1.6 and 2.5 for BH spins a * = 0.5 and 0.9, respectively, compared to a * = 0. Also, the density profile scales as ρ ( r ) ∝ r −1.1 for prograde BHs. Next, we considerably strengthen the initial magnetic field in the large torus by setting the plasma- β ≈ 1. This induces strong variability in the evolution. The jet efficiency in the a * = 0.9 model, for instance, now varies by over 3 orders of magnitude, and the gas rotation reverses directions. Combining these results with prior studies, we propose that a key parameter is the ratio R between the rotational and magnetic energies in the initial state. Strong variability appears later in models with larger values of R . The implication is that all simulations, and by extension all hot accretion flows in nature, will ultimately develop intermittent jets if evolved long enough.