Radiation GRMHD Models of Accretion onto Stellar-Mass Black Holes: I. Survey of Eddington Ratios

Lizhong Zhang, James M. Stone, Patrick D. Mullen, Shane W. Davis, Yan-Fei Jiang, Christopher J. White

First listed 2025-06-02 | Last updated 2025-09-12

Abstract

We summarize results from a survey of radiation-dominated black hole accretion flows across a wide range of mass accretion rates, as well as two values of black hole spin and initial magnetic field geometry. These models apply an algorithm targeting direct solutions to the radiation transport equation in full general relativity and have been enabled by access to modern exascale computing systems. Super-Eddington accretion flows form geometrically thick radiation pressure supported disks that drive powerful equatorial outflows. A narrow funnel-shaped photosphere in the inner region results in very low radiative efficiencies in this regime. The structure of near- and sub-Eddington accretion depends on whether there is net vertical magnetic flux at the midplane of the disk. With net flux, the disk forms a thin, dense layer at the midplane surrounded by a magnetically-dominated corona, whereas without net flux the disk remains magnetically dominated everywhere. Although none of our models achieve the magnetically arrested disk (MAD) regime, those with net vertical flux and a rapidly spinning black hole still produce powerful relativistic jets. Our calculations adopt simple opacity models (with scalings appropriate to stellar-mass black hole accretion). We discuss the application of our results to observations of X-ray binaries and ultraluminous X-ray sources such as Cyg X-3 and SS433. We also speculate on the application of our super-Eddington models to the interpretation of little red dots (LRDs) recently discovered by JWST.

Short digest

Exascale AthenaK GRRMHD simulations solve full radiation transport to survey stellar-mass black hole accretion from sub- to highly super-Eddington, spanning spins and initial magnetic topologies. Super-Eddington runs build geometrically thick, radiation-supported disks with a narrow inner funnel, drive powerful equatorial outflows, and remain radiatively inefficient; near/sub-Eddington structure hinges on net vertical flux—forming a thin dense midplane plus magnetically dominated corona when present, versus magnetically dominated throughout without it. Even short of MAD, net-flux high-spin cases launch relativistic jets, connecting to ULXs (Cyg X-3, SS433) and offering a framework for interpreting JWST little red dots. Caveat: results use simplified opacities tailored to stellar-mass systems.

Key figures to inspect

• Figure 2 — Use the accretion-rate and normalized magnetic-flux histories to identify the gray steady-state windows and to gauge proximity to the MAD limit; compare black (no-radiation) and radiative runs versus spin to see how radiation modifies flux accumulation.
• Figure 3 — Compare poloidal density slices across E88-a3, E08-a3, and E01-a3 to see how disk thickness, funnel opening angle, and midplane density morph with Eddington ratio; the bottom-row zoom highlights the inner funnel that sets beaming and low efficiency.
• Figure 4 — Inspect the decomposition into disk, wind, and jet using optical depth per r_g (colors), the cyan Bernoulli=0 contour (bound vs unbound), and orange jet boundaries to see where jets originate and how super- vs near-Eddington cases differ.
• Figure 1 — Check how single- vs double-loop magnetic initial conditions seed (or suppress) net vertical flux at the midplane, setting the later thin midplane layer plus corona versus everywhere-magnetized regimes.