Ab initio simulations of accretion disc instability

We show that accretion discs, both in the subcritical and supercritical accretion rate regime, may exhibit significant amplitude luminosity oscillations. The luminosity time behaviour has been obtained by performing a set of time-dependent two-dimensional smoothed particle hydrodynamics simulations of accretion discs with different values of alpha and accretion rate. In this study, to avoid any influence of the initial disc configuration, we produced the discs injecting matter from an outer edge far from the central object. The period of oscillations is 2-50 s for the two cases, and the variation amplitude of the disc luminosity is 10(38)-10(39) erg s(-1). An explanation of this luminosity behaviour is proposed in terms of limit cycle instability; the disc oscillates between a radiation pressure dominated configuration (with a high luminosity value) and a gas pressure dominated one (with a low luminosity value). The origin of this instability is the difference between the heat produced by viscosity and the energy emitted as radiation from the disc surface (the well-known thermal instability mechanism). We support this hypothesis showing that the limit cycle behaviour produces a sequence of collapsing and refilling states of the innermost disc region.