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

Abstract

We present two general relativistic radiation magnetohydrodynamics (GRRMHD) simulations of magnetically arrested disks (MADs) around non-spinning ($a_{\ast }=0$) and spinning ($a_{\ast }=0.9$) 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 $3\gtrsim \dot{M}/{\dot{M}}_{\mathrm{Edd}}\gtrsim 0.3$. For the non-spinning BH model, the total and radiative efficiencies increase as the accretion rate decreases, varying over the range $\hat{I}{\cdot }_{\mathrm{tot}}\sim 9-16\%$ and $\hat{I}{\cdot }_{\mathrm{rad}}\sim 6-12\%$, 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 $\hat{I}{\cdot }_{\mathrm{jet}}\sim 18-39\%$ while the total and radiative efficiencies are $\hat{I}{\cdot }_{\mathrm{tot}}\sim 64-100\%$ and $\hat{I}{\cdot }_{\mathrm{rad}}\sim 45-79\%$, 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 $100GM/c^2$ 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 $a_{\ast }=0.9$ model shows semi-regular variations with a period of $\sim 2000GM/c^3$ over the final $\sim 10,000GM/c^3$ of the simulation, which suggests that magnetic flux eruptions may be an important source of quasi-periodic variability. For the simulated accretion rates, the $a_{\ast }=0$ model is spinning up while the $a_{\ast }=0.9$ model is spinning down. Spinup-spindown equilibrium of the BH will likely be achieved at $0.5<a_{\ast ,\mathrm{eq}}<0.6$, assuming continuous accretion in the MAD state.

23 pages, 18 figures