Stability of slim accretion discs – Effects of central mass and viscosity

Abstract

Slim accretion discs have a total luminosity of the order |$L/L_{E}=\dot{m}\sim 1$|⁠, where LE is the Eddington luminosity and |$\dot{m}=\dot{M}/\dot{M}_{c}$|⁠, where Me is a critical accretion rate, related to the Eddington one. The local stability properties of such discs are examined, in the three-dimensional parameter space spanned by the (α, μ, m) axes, where α and μ are two viscosity parameters, and m = M/M⊙ the central mass. We suggest that various types of observed quasi-periodic behaviour may be connected with slim disc instabilities. If this turns out to be correct, the so-called normal and horizontal branch oscillations could be due to unstable thermal and acoustic modes, respectively. It is subsequently shown that some of the observed short-term (quasi-periodic) variability in active galactic nuclei may also originate from short-wavelength acoustic modes in the innermost region of the disc. Consequently, observational characteristics, in connection with stability theory, may yield estimates of basic accretion parameters. In the case of the Seyfert galaxy NGC6814, this process seems to favour |$(\alpha ,\mu ,m,\dot{m})\simeq (0.5,0,{10}^{6},{10}^{-2})$|⁠. We finally conclude that this line of work may provide additional evidence for both accretion discs and black holes, in various compact sources.