Pion production in the inner disk around Cygnus X-1
  Neutron production via ^{4}He breakup and p(p, n{pi}^+)p is considered in the innermost region of an accretion disk surrounding a Kerr Black Hole. These reactions occur in a plasma in Wien equilibrium, where (radiatively produced) pair production equals annihilation. Cooling of the disk is assumed to be due to unsaturated inverse Comptonization of external soft photons and to the energy needed to ignite both nuclear reactions. Assuming matter composition of 90% Hydrogen and 10% He, it is shown that, close to the border of this region, neutron production is essentially from ^{4}He breakup. Close to the horizon, the contribution from p(p, n{pi}^+)p to the neutron production is comparable to that from the breakup. It is shown that the viscosity generated by the collisions of the accreting matter with the neutrons may drive stationary accretion, for accretion rates below a critical value. In this case, solution to the disk equations is double-valued and for both solutions protons overnumber the pairs. It is claimed that these solutions may mimic the states of high and low luminosity observed in Cygnus X-1 and related sources. This would be explained either by the coupling of thermal instability to the peculiar behavior of the viscosity parameter alpha with the ion temperature that may intermittently switch accretion off or by the impossibility of a perfect tuning for both thermal and pair equilibrium in the disk, a fact that forces the system to undergo a kind of limit cycle behavior around the upper solution. 
Cesar Meirelles Filho, Celso L. Lima, Hideaki Miyake, and Varese   Timoteo