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Let's see what my references have to say about this.
"Gravitational and kinetic energy of matter falling onto the central object is converted by dissipation to heat. Heat is partially radiated out, partially converted to work on the disc expansion and (in the case of BH accretion) partially lost inside the hole. The efficiency of accretion disc \eta is defined by L = \eta {\dot M}c^2, where L is the total luminosity (power) of the disc radiation. Sołtan gave a strong observational argument, confirmed and improved later by other authors, that the efficiency of accretion in quasars is \eta \approx 0.1. Note that the efficiency of thermonuclear reactions inside stars is about two orders of magnitude smaller."
http://www.scholarpedia.org/article/Accretion_discs/2._Basic_physics_of_accretion_discs
"In physics, dissipation embodies the concept of a dynamical system where important mechanical modes, such as waves or oscillations, lose energy over time, typically due to the action of friction or turbulence. The lost energy is converted into heat, raising the temperature of the system. Such systems are called dissipative systems.
For example, a wave that loses amplitude is said to dissipate. The precise nature of the effects depends on the nature of the wave: an atmospheric wave, for instance, may dissipate close to the surface due to friction with the land mass, and at higher levels due to radiative cooling."
http://en.wikipedia.org/wiki/Dissipation
Seems about right.
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