Local Dynamical Instabilities in Magnetized, Radiation Pressure–supported Accretion Disks

Abstract

We present a general linear dispersion relation that describes the coupled behavior of magnetorotational, photon bubble, and convective instabilities in weakly magnetized, differentially rotating accretion disks. We presume the accretion disks to be geometrically thin and supported vertically by radiation pressure. We fully incorporate the effects of a nonzero radiative diffusion length on the linear modes. In an equilibrium with a purely vertical magnetic field, the vertical magnetorotational modes are completely unaffected by compressibility, stratification, and radiative diffusion. However, in the presence of azimuthal fields, which are expected in differentially rotating flows, the growth rate of all magnetorotational modes can be reduced substantially below the orbital frequency. This occurs if diffusion destroys radiation sound waves on the length scale of the instability and the magnetic energy density of the azimuthal component exceeds the nonradiative thermal energy density. While sluggish in this case, the magnetorotational instability still persists and will still tap the free energy of the differential rotation. Photon bubble instabilities are generically present in radiation pressure-dominated flows where diffusion is present. We show that their growth rates are limited to a maximum value that is reached at short wavelengths where the modes may be viewed as unstable slow magnetosonic waves. We also find that vertical radiation pressure destabilizes upward-propagating fast waves, and that Alfvén waves can be unstable. These instabilities typically have smaller growth rates than the photon bubble/slow modes. We discuss how all these modes behave in various regimes of interest and speculate how they may affect the dynamics of real accretion disk flows.