GRRMHD simulations of MAD accretion discs declining from super-Eddington to sub-Eddington accretion rates

Abstract

ABSTRACT We present two general relativistic radiation magnetohydrodynamics (GRRMHD) simulations of magnetically arrested discs (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 $3 \gtrsim \dot{M}/\dot{M}_{\rm {Edd}}\gtrsim 0.3$. For the non-spinning BH model, the total and radiative efficiencies increase as the accretion rate decreases, varying over the range $\eta _{\rm {tot}}\sim 9\!-\!16{{\ \rm per\ cent}}$ and $\eta _{\rm {rad}}\sim 6{-}12{{\ \rm per\ cent}}$, 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 $\eta _{\rm {jet}}\sim 18{-}39{{\ \rm per\ cent}}$ while the total and radiative efficiencies are $\eta _{\rm {tot}}\sim 64{-}100{{\ \rm per\ cent}}$ and $\eta _{\rm {rad}}\sim 45{-}79{{\ \rm per\ cent}}$, respectively. We confirm that mildly sub-Eddington discs 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 ≈ 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 semiregular 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.