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

We use a relativistic ray-tracing code to calculate the light curves observedfrom a global general relativistic magneto-hydrodynamic simulation of anaccretion flow onto a Schwarzschild black hole. We apply three basic emissionmodels to sample different properties of the time-dependent accretion disk.With one of these models, which assumes thermal blackbody emission andfree-free absorption, we can predict qualitative features of the high-frequencypower spectrum from stellar-mass black holes in the "Thermal Dominant" state.The simulated power spectrum is characterized by a power law of index Gamma ~ 3and total rms fractional variance of <~ 2% above 10 Hz. For each emissionmodel, we find that the variability amplitude should increase with increasinginclination angle. On the basis of a newly-developed formalism for quantifyingthe significance of quasi-periodic oscillations (QPOs) in simulation data, wefind that these simulations are able to identify any such features with(rms/mean) amplitudes >~ 1 % near the orbital frequency at the inner-moststable orbit. Initial results indicate the existence of transient QPO peakswith frequency ratios of nearly 2:3 at a 99.9% confidence limit, but they arenot generic features because at any given time they are seen only from certainobserver directions. Additionally, we present detailed analysis of theazimuthal structure of the accretion disk and the evolution of densityperturbations in the inner disk. These "hot spot" structures appear to beroughly self-similar over a range of disk radii, with a single characteristicsize \delta\phi=25 deg and \delta r/r=0.3, and typical lifetimes T_l ~ 0.3T_orb.Comment: Accepted to ApJ, 43 pages, 13 figures, 2 table