Magneto-rotational instability in magnetically polarized discs

Abstract: The magneto-rotational instability (MRI) is the most likely mechanism for transportation of angular momentum and dissipation of energy within hot, ionized accretion discs. This instability is produced through the interactions of a differentially rotating plasma with an embedded magnetic field. Like all substances in nature, the plasma in an accretion disc has the potential to become magnetically polarized when it interacts with the magnetic field. In this paper, we study the effect of this magnetic susceptibility, parameterized by $\chi_m$, on the MRI, specifically within the context of black hole accretion. We find from a linear analysis within the Newtonian limit that the minimum wavelength of the first unstable mode and the wavelength of the fastest growing mode are shorter in paramagnetic ($\chi_m>0$) than in diamagnetic ($\chi_m<0$) discs, all other parameters being equal. Furthermore, the magnetization parameter (ratio of gas to magnetic pressure) in the saturated state should be smaller when the magnetic susceptibility is positive than when it is negative. We confirm this latter prediction through a set of numerical simulations of magnetically polarized black hole accretion discs. We additionally find that the vertically integrated stress and mass accretion rate are somewhat larger when the disc is paramagnetic than when it is diamagnetic. If astrophysical discs are able to become magnetically polarized to any significant degree, then our results would be relevant to properly interpreting observations.