Acreting black holes in binary systems often exhibit quasi-periodic oscillations (QPOs) of the observed X-rays. Sometime the frequency of these oscillations is very high (kilohertz) and they occur at two distinct peaks. QPO properties differ between sources, however, it appears that they keep a fixed frequency ratio of small rational numbers. The origin of this phenomenon is currently unknown.
High-frequency QPOs are only one representation of the rich family of QPOs present in accreting sources. In fact, X-ray binaries show variability of the radiation on a wide range of time scales. The most prominent phenomenon was the discovery, four decades ago, of aperiodic changes of the flux from low-mass X-ray binaries, nowadays known as high-frequency quasi-periodic oscillations. Scorpius X-1 is a prototypical example. These oscillations occur both in neutron-star and in black-hole sources, and they feature two related peaks in the power spectrum (a.k.a. twin-peak QPOs). High-frequency QPOs are extensively studied because their characteristic periods are close to dynamical time-scales for the motion under the influence of gravity within a few gravitational radii off a compact object. They could help us to understand accretion processes onto black holes and they could constrain properties of super-dense matter in circumstances that cannot be reproduced in our laboratories.
For good reasons the high-frequency QPOs can be considered as the most striking, and also the most mysterious type of cosmic oscillations.Most astrophysicists think that these oscillations are generated during the process of accretion and their frequencies corresponds to some periodically repeating events in the accretion disk. This idea is supported by remarkable similarity between the higher frequency and the orbital frequency in the inner part of the disk. Since the orbital frequencies scale as the inverse of mass, the frequencies of QPO in black holes candidates should scale accordingly, and this seems to be in a good agreement with current observation data.