5. Quasi-Periodic Oscillations

Quasi-periodic oscillations (QPOs) provide information about the inner accretion disk and its interaction with the magnetosphere. The beat frequency mass accretion model assumes that the QPO is the beat frequency between the magnetospheric rotation and clumps in the inner accretion disk. The mass accretion rate is enhanced when these clumps connect to open field lines, yielding a quasi-periodic modulation of the X-ray luminosity. A test of this theory is the prediction that the QPO frequency should be luminosity-dependent.

A new class of fast QPOs, called ``kilohertz QPOs'', has recently been discovered with NASA's RXTE satellite (e.g. van der Klis (1997)). These QPOs are probably associated with the rotation periods of neutron stars in LMXB. Photon bubble oscillations (Klein et al. (1996)) are another possible mechanism that could generate kilohertz QPOs in AXP. These might be produced by the formation of radiation-pressure dominated bubbles in a locally super-Eddington accretion column.