The Radiative Efficiency of Hot Accretion Flows

TL;DR

This paper systematically studies the radiative efficiency of hot accretion flows, considering recent advances like outflows and direct electron heating, revealing efficiency increases with accretion rate and is comparable to thin disks at high rates.

Contribution

It provides new fitting formulae for radiative efficiency of hot accretion flows across different accretion rates and electron heating fractions, incorporating recent progress in the field.

Findings

Radiative efficiency increases with accretion rate.

Efficiency is significantly enhanced by direct viscous heating to electrons.

At high accretion rates, efficiency approaches that of standard thin disks.

Abstract

Two significant progresses have been made in the past years on our understanding of hot accretion flows. One is that only a small fraction of accretion flow available at the outer boundary can finally falls onto the black hole while most of them is lost in outflow. Another one is that electrons may directly receive a large fraction of the viscously dissipated energy in the accretion flow, i.e, $\delta\sim 0.1-0.5$. The radiative efficiency of hot accretion flow when these two progresses are taken into account has not been systematically studied and is the subject of the present paper. We consider two regimes of hot accretion model. One is the advection dominated accretion flows (ADAFs) which lie on low accretion rate regime, $\la 10\alpha^2\ledd/c^2$; another being the luminous hot accretion flows (LHAFs) which lie above this accretion rate. For the latter, we assume that the accretion flow will has a two-phase structure above a certain accretion rate, and a simplification is adopted in our calculation of the dynamics. Our results indicate that the radiative efficiency of hot accretion flow increases with the accretion rate and is highly enhanced by the direct viscous heating to electrons compared to the previous case of $\delta\ll 1$. When the accretion rate is high, the radiative efficiency of hot accretion flow is comparable to that of the standard thin disk. Fitting formulae of radiative efficiency as a function of accretion rate for various $\delta$ values are presented.