GRRMHD simulations of MAD accretion discs declining from super-Eddington to sub-Eddington accretion rates. Issue 3 (16th November 2022)

• Record Type: Journal Article
• Authors: Curd, Brandon; Narayan, Ramesh
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 518:Issue 3(2023)
• Page Start: 3441
• Page End: 3461
• Publication Date: 2022-11-16
• Subjects: accretion, accretion discs -- black hole physics -- MHD -- radiative transfer -- gamma-rays: galaxies -- X-rays: galaxies; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac3330
• Languages: English
• ISSNs: 0035-8711

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.