The average luminosity can be calculated as E/delta t , where E is the
total amount of energy radiated by the accretion disk: E=mc^2. m is the mass of
the accretion disk, c is the the speed of light. Delta t is known from the
question delta t = 12 billion years. Unknown is the mass of the accretion disk
and this mass depends on the type of the accretion disk.
