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 theinnermost region of an accretion disk surrounding a Kerr Black Hole. Thesereactions occur in a plasma in Wien equilibrium, where (radiatively produced)pair production equals annihilation. Cooling of the disk is assumed to be dueto unsaturated inverse Comptonization of external soft photons and to theenergy needed to ignite both nuclear reactions. Assuming matter composition of90% 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 tothat from the breakup. It is shown that the viscosity generated by thecollisions of the accreting matter with the neutrons may drive stationaryaccretion, for accretion rates below a critical value. In this case, solutionto the disk equations is double-valued and for both solutions protonsovernumber the pairs. It is claimed that these solutions may mimic the statesof high and low luminosity observed in Cygnus X-1 and related sources. Thiswould be explained either by the coupling of thermal instability to thepeculiar behavior of the viscosity parameter alpha with the ion temperaturethat may intermittently switch accretion off or by the impossibility of aperfect tuning for both thermal and pair equilibrium in the disk, a fact thatforces the system to undergo a kind of limit cycle behavior around the uppersolution.Comment: 23 pages, 11 figure