Quasi periodic oscillations in black hole binaries
  Fast time variability is the most prominent characteristic of accreting
systems and the presence of quasi periodic oscillations (QPOs) is a constant in
all accreting systems, from cataclysmic variables to AGNs, passing through
black hole and neutron star X-ray binaries and through the enigmatic
ultra-luminous X-ray sources. In this paper I will briefly review the current
knowledge of QPOs in black hole X-ray binaries, mainly focussing on their
observed properties, but also mentioning the most important models that have
been proposed to explain the origin of QPOs over the last decades.
