Radiation GRMHD Models of Accretion onto Stellar-Mass Black Holes: III. Near-Eddington Accretion

TL;DR

This paper models near-Eddington black hole accretion using GRMHD with radiation, revealing stable disk solutions influenced by magnetic flux and spin, and analyzing jet and wind properties.

Contribution

It introduces comprehensive GRMHD radiation models showing how magnetic flux and spin determine accretion disk states and jet formation near Eddington luminosity.

Findings

Two stable accretion solutions: thin disk and magnetically elevated disk.

Radiative cooling efficiency of 4-10%.

Jets and winds depend on magnetic flux and spin.

Abstract

We present a comprehensive analysis of four near-Eddington black hole accretion models computed by solving the GRMHD equations with full radiation transport. This study focuses on the dynamical effects of magnetic field topology and black hole spin. Two stable near-Eddington solutions emerge in these models: a thin thermal disk embedded within a magnetic envelope when sufficient net vertical magnetic flux is present (e.g., vertical field $\gtrsim 5\times10^5$ G at $20r_g$), and a magnetically elevated disk when the net vertical flux is weak or absent. One model initialized without net vertical flux is found to evolve into the thin disk solution, as strong, anisotropic radiation feedback at high accretion rates promotes the accumulation of vertical magnetic flux in the inner disk. In the thin thermal disk, accretion is driven primarily by mean-field Maxwell stress and proceeds largely within the magnetic envelope, while heat dissipation is spatially decoupled and concentrated near the midplane. However, in the magnetically elevated disk, accretion occurs throughout the disk body and is comparably driven by mean-field and turbulent stresses; heat dissipation therefore occurs locally through turbulence. Radiation transport is diffusion-dominated, enabling efficient radiative cooling ($\sim$4-10%). An optically thin wind is launched from the disk surface by combined radiative and magnetic forces, with its strength increasing with black hole spin and vertical magnetic flux. Both strong and weak jets are produced in these models: strong jets are persistent, highly relativistic, and magnetically driven, while weak jets are intermittent, mildly relativistic, and powered by a combination of magnetic and radiative forces.