GRRMHD Simulations of MAD Accretion Disks Declining from Super-Eddington to Sub-Eddington Accretion Rates

Brandon Curd; Ramesh Narayan

doi:10.1093/mnras/stac3330

paper

GRRMHD Simulations of MAD Accretion Disks Declining from Super-Eddington to Sub-Eddington Accretion Rates

arXiv:2209.12081 · doi:10.1093/mnras/stac3330

Abstract

We present two general relativistic radiation magnetohydrodynamics (GRRMHD) simulations of magnetically arrested disks (MADs) around non-spinning ( $a_{*} = 0$ ) and spinning ( $a_{*} = 0.9$ ) supermassive black holes (BHs). In each simulation, the mass accretion rate is decreased with time such that we sample Eddington-scaled rates over the range $3 ≳ M ˙ / M ˙_{Edd} ≳ 0.3$ . For the non-spinning BH model, the total and radiative efficiencies increase as the accretion rate decreases, varying over the range $η_{tot} \sim 9 - 16%$ and $η_{rad} \sim 6 - 12%$ , respectively. This model shows very little jet activity. In contrast, the spinning BH model has a strong relativistic jet powered by spin energy extracted from the BH. The jet power declines with accretion rate such that $η_{jet} \sim 18 - 39%$ while the total and radiative efficiencies are $η_{tot} \sim 64 - 100%$ and $η_{rad} \sim 45 - 79%$ , respectively. We confirm that mildly sub-Eddington disks can extract substantial power from a spinning BH, provided they are in the MAD state. The jet profile out to $100 GM / c^{2}$ is roughly parabolic with a power-law index of $k \approx 0.43 - 0.53$ during the sub-Eddington evolution. Both models show significant variability in the outgoing radiation which is likely associated with episodes of magnetic flux eruptions. The $a_{*} = 0.9$ model shows semi-regular variations with a period of $\sim 2000 GM / c^{3}$ over the final $\sim 10, 000 GM / c^{3}$ of the simulation, which suggests that magnetic flux eruptions may be an important source of quasi-periodic variability. For the simulated accretion rates, the $a_{*} = 0$ model is spinning up while the $a_{*} = 0.9$ model is spinning down. Spinup-spindown equilibrium of the BH will likely be achieved at $0.5 < a_{*, eq} < 0.6$ , assuming continuous accretion in the MAD state.

arXiv comment23 pages, 18 figures

GRRMHD Simulations of MAD Accretion Disks Declining from Super-Eddington to Sub-Eddington Accretion Rates

Abstract

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