Light Curves from an MHD Simulation of a Black Hole Accretion Disk

Abstract

We use a relativistic ray-tracing code to calculate the light curves observed from a global general relativistic magneto-hydrodynamic simulation of an accretion flow onto a Schwarzschild black hole. We apply three basic emission models to sample different properties of the time-dependent accretion disk. With one of these models, which assumes thermal blackbody emission and free-free absorption, we can predict qualitative features of the high-frequency power spectrum from stellar-mass black holes in the "Thermal Dominant" state. The simulated power spectrum is characterized by a power law of index Gamma ~ 3 and total rms fractional variance of <~ 2% above 10 Hz. For each emission model, we find that the variability amplitude should increase with increasing inclination angle. On the basis of a newly-developed formalism for quantifying the significance of quasi-periodic oscillations (QPOs) in simulation data, we find that these simulations are able to identify any such features with (rms/mean) amplitudes >~ 1 % near the orbital frequency at the inner-most stable orbit. Initial results indicate the existence of transient QPO peaks with frequency ratios of nearly 2:3 at a 99.9% confidence limit, but they are not generic features because at any given time they are seen only from certain observer directions. Additionally, we present detailed analysis of the azimuthal structure of the accretion disk and the evolution of density perturbations in the inner disk. These "hot spot" structures appear to be roughly self-similar over a range of disk radii, with a single characteristic size δϕ=25 deg and δr/r=0.3, and typical lifetimes T_l ~ 0.3 T_orb.