On the stability of shocks in isothermal black hole accretion discs (original) (raw)

Most black holes possess accretion discs. Models of such discs inform observations and constrain the properties of the black holes and their surrounding medium. Here, we study shocks in a thin isothermal black hole accretion flow. Modelling infinitesimal viscosity allows the use of multiple-scales matched asymptotic methods. We thus derive the first explicit calculations of isothermal shock stability. We find that the inner shock is al w ays unstable, and the outer shock is al w ays stable. The growth/decay rates of perturbations depend only on an ef fecti ve potential and the incoming-outgoing flo w dif ference at the shock location. We give a prescription of accretion regimes in terms of angular momentum and black hole radius. Accounting for viscous angular momentum dissipation implies unstable outer shocks in much of parameter space, even for realistic viscous Reynolds numbers of the order ≈10 20 .