Radiatively inefficient accretion flow (RIAF)

A radiatively inefficient accretion flow carries a substantial fraction of locally generated energy with the gas instead of radiating it before accretion. It can occur at low density because emission is weak or at high optical depth because photons are trapped.

An advection-dominated accretion flow is a hot, geometrically thick, radiatively inefficient flow in which most dissipated energy remains in the gas and is carried inward rather than radiated locally.

A two-temperature accretion flow has ions and electrons at substantially different temperatures because their Coulomb equilibration time exceeds the inflow time. Ions can retain most viscous heat while electrons, which radiate more effectively, remain cooler.

The electron--proton thermal equilibration time is the timescale on which Coulomb collisions exchange energy between electron and proton populations. Its inverse dependence on number density helps make dilute accretion flows two-temperature and radiatively inefficient.

A slim accretion disk is an optically thick, moderately geometrically thick flow at a high accretion rate. Its inflow time can be shorter than its photon-diffusion time, so radial advection carries trapped radiation inward.

Photon trapping occurs when the radiative diffusion time across an optically thick flow exceeds its inward advection time. The trapped radiation is carried inward, reducing the emergent radiative efficiency.

A luminous hot accretion flow has radiative cooling greater than local viscous heating, so compressive inward advection supplies the difference. In the convention $q_{\mathrm{adv}}=q^{+}-q^{-}$, it has $q_{\mathrm{adv}}<0$.

The accretion-flow advection balance compares local volumetric heating $q^{+}$, radiative cooling $q^{-}$, and energy transport with the gas. Positive $q_{\mathrm{adv}}$ means advection removes locally generated heat; zero gives local radiative balance; negative means advection supplies heat that is radiated locally.