Pion Production in the Inner Disk around Cygnus X‐1

Abstract

Neutron production via 4He breakup and p(p, nπ+)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% H and 10% He, we show that, close to the border of this region, neutron production is essentially from 4He breakup. Close to the horizon, the contribution from p(p, nπ+)p to the neutron production is comparable to that from the breakup. We show 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, the solution to the disk equations is double-valued, and for both solutions protons outnumber the pairs. Our results apply whenever 17/M < 1.74. We suggest that these solutions may mimic the states of high and low luminosity observed in Cygnus X-1 and related sources. This could be explained either by the coupling of thermal instability to the peculiar behavior of the viscosity parameter α 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.