High-frequency QPO modulation in neutron-star and black-hole sources

After twelve years have past since the discovery of the first high-frequency quasi-periodic oscillation in Sco X-1, an enormous progress has been made on the observational side and on the field of data analysis. Equal amount of effort has been devoted also to the theoretical aspect of the QPO problem. However, so far no model is able to sufficiently fully explain the mechanism of QPO production. There are two aspects: the question of the QPO origin; and the question of its modulation. In this article, I am focusing solely on the second issue - I discuss ways of how the outgoing flux can be modulated at a level sufficient to match the observational rms amplitudes. Some more space is devoted to my own work in the field.