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

Abstract

An analysis of two-dimensional viscous, radiation hydrodynamic numerical simulations of thin α-disks around a stellar mass black hole reveals multiple robust, coherent oscillations. Our disk 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 LEdd/c2. In the initially radiation-pressure-dominated disk, 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 disk 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 disks strongly suggest standing-wave p-modes with frequencies below the apparent g-mode frequency, some trapped in the inner disk close to the ISCO, others present in the middle/outer parts of the disk. 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.

Disk Tearing Leads to Low and High Frequency Quasi Periodic Oscillations in a GRMHD Simulation of a Thin Accretion Disk

“…The radially extended regions of high power along the radial epicyclic curve shown in Figure 5 have similarly been found in the PSDs (e.g. of the fluid three velocity) generated from the viscous, radiative hydrodynamic 2D simulation of aligned thin disks around a Schwarzschild black hole analysed in Mishra et al (2019Mishra et al ( , 2020. In Mishra et al (2019) they find radially extended regions of high power along the radial epicyclic curve.…”

“…In Mishra et al (2019) they find radially extended regions of high power along the radial epicyclic curve. Mishra et al (2020) also report local enhancements of the variability manifested as "stripes" in the PSDs, one of which is identified as a trapped g-mode (their Figures 1&4). These features are promising, however it remains to be seen which of the numerous modes are robust against damping by the magnetorotational-driven turbulence.…”

Radiation Transport Two-temperature GRMHD Simulations of Warped Accretion Disks

“…While radiation-pressure-supported accretion disks in GRMHD simulations typically collapse (e.g., Saḑowski 2016;Fragile et al 2018;Jiang et al 2019;Mishra et al 2020;Liska et al 2022a) into an infinitely thin slab due to thermal instability (Shakura & Sunyaev 1976), this did not happen in our simulation. Here we argue that the warp and associated nozzle shock stabilize the disk against thermal collapse (Section 5).…”

Lense–Thirring precession after a supermassive black hole disrupts a star

“…Red noise is a common type of variability noise that is predicted from both general relativistic magnetohydrodynamic simulations (see, for example, ref. 47 ) and X-ray observations and is, in general, described analytically by a power law or a bending power law 48 . We rebinned the LSP by a factor of 10 and fitted it with a power-law + constant and a bending power-law + constant models.…”