Sheperd Doeleman said in a TED video that the accretion disk is heated by friction. Is that correct? I thought it might be adiabatic heating from the compression of the gasses.
It is mainly caused by:
Thomson scattering (coherent)
Compton scattering (decoherent)
Please see here:
I see no reason why both effects could not contribute to the heating,one does not exclude the other. Collisions between particles in the accretion disc must make some contributon,but that probably comes under the heading of friction. If there are radioactive materials in the accretion disc,that must also make a contribution,however small.