Dong LaiWen Fu, Jiri Horak, Ryan Miranda, David Tsang, Cong Yu30 or more min

Excitations of Global Oscillations in Black-Hole Accretion Disks and the Physical Origin of High-Frequency QPOs

The physical origin of high-frequency QPOs in black-hole X-ray binaries remains an enigma despite many years of detailed observational studies. Although there exist a number of "models" for HFQPOs, many of these are notions/concepts (e.g., a hot spot orbiting at some specific radius) which, while appealing, do not yet derive from actual calculations based on self-consistent disk physics. It is hoped that LOFT will greatly enhance the phenomenology of HFQPOs. On the theoretical sides, future progress will require a combination of numerical simulations and semi-analytic studies to extract physical insights. I will discuss recent works on global oscillation modes in black-hole accretion flows from the Cornell group and other related efforts, and explain how, with the help of general relativity, the energy stored in the disk differential rotation can be pumped into global spiral density modes in the disk, making these modes grow naturally to large amplitudes under certain conditions ("corotational instability"). These modes are robust in the presence of disk magnetic fields and turbulence. The success and challenge of the simplest version of the disk models will be examined and reviewed. The connection of HFQPOs with other properties of X-ray binaries (such as production of episodic jets) will also be discussed.