= Black-hole accretion
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Black-hole accretion is the inflow of matter onto a <black hole>, converting some of its <gravitational binding energy> into <heat>, <electromagnetic radiation> and outflows. Matter with substantial <angular momentum> generally forms an <accretion disk>; internal <stresses> transfer <angular momentum> outward so that matter can move inward. The released <energy> can be radiated before the material crosses the <event horizon>. In a thin-disk model the <innermost stable circular orbit> marks the transition toward rapid plunging motion, although the detailed inner flow depends on the accretion state.<Dynamics of Astrophysical Discs, Lecture 5: Boundary Conditions and Steady Accretion by Gordon I. Ogilvie>[Sections 5.1–5.2: inner black-hole boundary, angular-momentum transport and steady-disk luminosity]{ref}
The relation $L=\eta\dot M c^2$ defines the <radiative efficiency of black-hole accretion>, where $L$ is emitted <luminosity>, $\dot M$ the <mass accretion rate> and $c$ the <speed of light>. A <radiatively inefficient accretion flow> carries much of its available <energy> inward or into outflows instead of emitting it, so luminosity alone need not measure the mass supply.
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