Development of Secular Instability in Different Disc Models of Black Hole Accretion

Analytical treatment of black hole accretion generally presumes the stability of the stationary configuration. Various authors in past decade actually demonstrated the validity of such an assumption at least for inviscid hydrodynamic flow. Inviscid condition is a reasonable approximation for low angular momentum advection dominated flow in connection to certain supermassive black holes at the centres of the galaxies (including our own) fed from a number of stelar donors. Introduction of a weak viscosity, however, may sometimes provide a more detail understanding of the observed spectrum. But recently, some of the authors demonstrated that an introduction of slightest amount of viscosity in quasi-viscous form makes a stationary accretion disc (in vertical equilibrium model) unstable. We made the same sort of analysis in other disc models for quasi-viscous models under post-Newtonian scheme. We introduced perturbations on the stationary flow solution particularly in standing wave form and studied their time evolution to observe whether they grow with time. Our analysis shows that same sort of secular instability exists in other disc models too. We further argued that with sufficiently low value of viscosity in the realistic Astrophysical perspective the instability does not effectively jeopardise the stationary condition.