Relativistic, axisymmetric, viscous, radiation hydrodynamic simulations of geometrically thin discs. II. Disc variability

Abstract

ABSTRACT An analysis of two-dimensional viscous, radiation hydrodynamic numerical simulations of thin α-discs around a stellar mass black hole reveals multiple robust, coherent oscillations. Our disc models are initialized on both the gas- and radiation-pressure-dominated branches of the thermal equilibrium curve, with mass accretion rates between $\dot{M} = 0.01 L_\mathrm{Edd}/c^2$ and $10\, L_\mathrm{Edd}/c^2$. In the initially radiation-pressure-dominated disc, we confirm the presence of global inertial–acoustic oscillations of frequency slightly above the maximum radial epicyclic one. In the gas-pressure-dominated Schwarzschild-metric models, we find a velocity oscillation occurring at the maximum value of the radial epicyclic frequency, $3.5\times 10^{-3}\, t_\mathrm{g}^{-1}$, which is most likely a trapped fundamental g-mode. For the Kerr-metric, gas-pressure-dominated disc with dimensionless black hole spin parameter a* = 0.5, the mode frequency is well below the epicyclic frequency maximum, thus confirming that this oscillation is a trapped g-mode. Additionally, the total pressure fluctuations in the discs strongly suggest standing-wave p-modes with frequencies below the apparent g-mode frequency, some trapped in the inner disc close to the innermost stable circular orbit (ISCO), others present in the middle/outer parts of the disc. The strongest oscillations occur at the breathing oscillation frequency and are present in all the numerical models we report here, as are weaker velocity oscillations at the vertical epicyclic frequencies. The vertical oscillations show a 3:2 frequency ratio with oscillations occurring approximately at the radial epicyclic frequency, which could be of astrophysical importance in systems with observed twin peak, high-frequency quasi-periodic oscillations.