Radiation GRMHD Models of Accretion onto Stellar-Mass Black Holes: II. Super-Eddington Accretion

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

First listed 2025-09-12 | Last updated 2026-03-05

Abstract

We present a comprehensive analysis of super-Eddington black hole accretion simulations that solve the GRMHD equations coupled with angle-discretized radiation transport. The simulations span a range of accretion rates, two black hole spins, and two magnetic field topologies, and include resolution studies as well as comparisons with non-radiative models. Super-Eddington accretion flows consistently develop geometrically thick disks supported by radiation pressure, regardless of magnetic field configuration. Radiation generated in the inner disk drives substantial outflows, forming conical funnel regions that limit photon escape and result in very low radiation efficiency. The accretion flows are highly turbulent with thermal energy transport dominated by radiation advection rather than diffusion. Angular momentum is primarily carried outward by Maxwell stress, with turbulent Reynolds stress playing a subdominant role. Both strong and weak jets are produced. Strong jets arise from sufficient net vertical magnetic flux and rapid black hole spin and can effectively evacuate the funnel, enabling radiation to escape through strong geometric beaming. In contrast, weak jets fail to clear the funnel, which becomes obscured by radiation-driven outflows and leads to distinct observational signatures. Spiral structures are observed in the plunging region, behaving like density waves. These super-Eddington models are applicable to a variety of astronomical systems, including ultraluminous X-ray sources, little red dots, and black hole transients.

Short digest

A radiation–GRMHD suite with angle-discretized transport follows super‑Eddington inflow across accretion rates, spins a*≈0.3–0.94, and single/double‑loop fields, finding robust, radiation‑pressure supported thick disks. Inner‑disk radiation drives powerful outflows that carve conical funnels, trap photons, and keep radiative efficiencies low while thermal transport is dominated by advection. Angular momentum is carried mainly by Maxwell stress; jets occur in weak and strong modes, with strong jets (net vertical flux + high spin) evacuating the funnel and enabling geometrically beamed escape, whereas weak jets leave the funnel clogged and dimmer—implications for ULXs and little red dots. Spiral density waves appear in the plunging region, adding structure to the inner flow.

Key figures to inspect

• 3D renderings of model E88‑a3‑LR (gas density + magnetic streamlines; radiation energy density): inspect jet collimation, funnel opening angle, and how helical fields coincide with the evacuated core that enables beamed escape.
• Table 1 (model comparison): read off Δz_disk@10 rg, inflow‑equilibrium radius r_eq, photon‑trapping radius r_tr, and the power‑law fits for thermal/magnetic pressure and Maxwell/Reynolds stresses to see how spin and topology shift trapping and transport.
• Radial transport budgets (time/φ averages): identify where radiation advection overtakes diffusion and quantify the Maxwell‑dominated angular‑momentum flux versus subdominant turbulent Reynolds stress.
• Funnel optical depth and radiation flux contrasts between strong‑jet and weak‑jet runs: verify when net vertical flux + high spin clear the funnel, producing narrow beaming and higher apparent luminosity versus wind‑obscured funnels.
• Plunging‑region maps: track spiral density waves inside r_ISCO, their pattern relative to inflow, and their imprint on variability channels.