Black hole spin gone MAD in radiatively-inefficient and luminous disks

Supermassive black holes interact with their host galaxies by accreting gas and expelling energy into the surrounding medium. This interaction takes the form of relativistic jets, which are observed in 1 out of every 10 active galaxies. Understanding the feedback of jets on the galaxy is crucial for modeling galaxy evolution.

Since the jets are thought to be powered by black hole spin, radio observations of jets may provide insight to the underlying physics near the black hole. However, how black hole spin evolves as black holes accrete and produce jets is not understood. I have performed 3D general relativistic magnetohydrodynamic (GRMHD) simulations of magnetically-saturated, or magnetically arrested disk (MAD), accretion onto black holes. I used GRMHD simulations of nonradiative disks representative of super-Eddington accretion for a wide range of dimensionless black hole spin (-0.9 < a < 0.99). I then used radiation transport two-temperature GRMHD simulations of luminous sub-Eddington disks to discern how the results depend on the Eddington ratio.

I show that MADs spin down the black holes to low spin values of 0.1 for radiatively-inefficient super-Eddington disks. I then construct a semi-analytic model and use it to demonstrate that the low value of equilibrium spin emerges primarily due to the combination of a powerful jet-induced magnetic torque on the black hole and a highly sub-Keplerian disk near the black hole. I then show that luminous MADs spin down black holes less, leading to a higher equilibrium spin, a = 0.4. This signals a paradigm shift that luminous jetted quasars can spin down the black holes to rather low values of spin, a = 0.4, much lower than the equilibrium spin value expected for standard luminous disks, a = 0.998. Finally, I use my semi-analytic model to reveal why luminous disks result in less black hole spin-down than radiatively-inefficient MADs.