Graduate Student Seminar with Andrew West on Accretion Flows Around Black Holes

While Quasi-Periodic Oscillations (QPOs) in the emission from Black Hole X-Ray binaries have been studied for decades, their physical origin has remained elusive. One possible explanation is that the variations in flux are caused by the precession of the inner regions of the accretion flow. Liska et al. (2019) have developed the state-of-the-art GPU accelerated General Relativistic Magnetohydrodynamical (GRMHD) code H-AMR, which utilizes adaptive mesh refinement to achieve the highest possible fidelity in regions of the disk with high dynamic variability. They have shown that an initially misaligned disk may under-go numerous tearing events which split the accretion flow into multiple distinct sub-disks, which may then precess at different frequencies. Using xTrack, a forward integrating raytracing code originally developed by Henric Krawczynski (2012), we post-process this dataset. We model the thermal emission from the disk and its subsequent scattering off of the evolving disk structure, utilizing the full runtime of the simulation from initial misalignment, through multiple tearing events and precession periods, to a final semi-stable Bardeen-Petterson configuration. Our results show the presence of multiple QPOs, sometimes occurring at single frequencies, or pairs and even triplets, representing 2:1 And 3:2 frequency ratios. Interestingly, while the characteristic frequency of these signals is related to the evolution of the inner regions of the flow, we show that these signals are in fact global modes emanating from the inner and outer regions of the disk.