Hyper-Eddington mass accretion onto a black hole with super-Eddington luminosity

TL;DR

This study uses radiation hydrodynamical simulations and analytic modeling to explore conditions under which black holes can sustain hyper-Eddington accretion rates, even with super-Eddington luminosities, revealing a critical luminosity threshold for steady accretion.

Contribution

It demonstrates that hyper-Eddington accretion can occur with luminosities exceeding the Eddington limit, identifying the conditions and thresholds for steady versus episodic accretion.

Findings

Steady hyper-Eddington accretion occurs below a specific luminosity threshold.

Accretion becomes episodic when luminosity exceeds the critical value.

A toy model effectively reproduces the simulation results.

Abstract

We perform one-dimensional radiation hydrodynamical simulations to solve accretion flows onto massive black holes (BHs) with a very high rate. Assuming that photon trapping limits the luminosity emerging from the central region to $L\lesssim L_{\rm Edd}$, Inayoshi, Haiman & Ostriker (2016) have shown that an accretion flow settles to a "hyper-Eddington" solution, with a steady and isothermal ($T\simeq 8000$ K) Bondi profile reaching $\gtrsim 5000$ times the Eddington accretion rate $\dot{M}_{\rm Edd}\equiv L_{\rm Edd}/c^2$. Here we address the possibility that gas accreting with finite angular momentum forms a bright nuclear accretion disc, with a luminosity exceeding the Eddington limit ($1\lesssim L/L_{\rm Edd}\lesssim 100$). Combining our simulations with an analytic model, we find that a transition to steady hyper-Eddington accretion still occurs, as long as the luminosity remains below $L/L_{\rm Edd}\lesssim 35~(M_{\rm BH}/10^4~M_\odot)^{3/2}(n_\infty/10^5~{\rm cm^{-3}})(T_\infty/10^4~{\rm K})^{-3/2}(r_{\star}/10^{14}~{\rm cm})^{-1/2}$, where $n_\infty$ and $T_\infty$ are the density and temperature of the ambient gas, and $r_\star$ is the radius of the photosphere, at which radiation emerges. If the luminosity exceeds this value, accretion becomes episodic. Our results can be accurately recovered in a toy model of an optically thick spherical shell, driven by radiation force into a collapsing medium. When the central source is dimmer than the above critical value, the expansion of the shell is halted and reversed by ram pressure of the collapsing medium, and by shell's weight. Our results imply that rapid, unimpeded hyper-Eddington accretion is possible even if the luminosity of the central source far exceeds the Eddington limit, and can be either steady or strongly episodic.