Trapped, Two-Armed, Nearly Vertical Oscillations in Disks with Toroidal Magnetic Fields

Abstract

We have examined trapping of two-armed ($m=2$) nearly vertical oscillations (vertical p-mode) in vertically isothermal ($c_{\rm s}=$ const.) relativistic disks with toroidal magnetic fields. The magnetic fields are stratified so that the Alfv{\'e}n speed, $c_{\rm A}$, is constant in the vertical direction. The ratio of $c_{\rm A}^2/c_{\rm s}^2$ in the vertical direction is taken as a parameter examining the effects of magnetic fields on wave trapping. We find that the two-armed nearly vertical oscillations are trapped in the inner region of disks and their frequencies decrease with increase of $c_{\rm A}^2/c_{\rm s}^2$. The trapped regions of the fundamental ($n=1$) and the first-overtone ($n=2$) are narrow (less than the length of the Schwarzschild radius, $r_{\rm g}$) and their frequencies are relatively high (on the order of the angular frequency of disk rotation in the inner region). On contrast to this, the second-overtone ($n=3$) are trapped in a wide region (a few times $r_{\rm g}$), and their frequencies are low and tend to zero in the limit of $c_{\rm A}^2/c_{\rm s}^2=2.0$.