Outbursts of Irradiated Accretion Discs

I solve analytically the viscous evolution of a self–irradiated accretion disc, as seen during outbursts of soft X–ray transients. The solutions predict steep power–law X– ray decays LX ∼ (1 + t/tvisc) , changing to LX ∼ (1 − t/t ′ visc ) at late times, where tvisc, t ′ visc are viscous timescales. These forms closely resemble the approximate exponential and linear decays inferred by King and Ritter (1997) in these two regimes. The decays are much steeper than for unirradiated discs because the viscosity is a function of the central accretion rate rather than of local conditions in the disc.