section_path: Observational evidence/Accretion of matter/X-ray binaries/Quasi-periodic oscillations
section_name: Quasi-periodic oscillations
paragraph_text: Quasi-periodic oscillations
  
  The X-ray emissions from accretion disks sometimes flicker at certain frequencies. These signals are called quasi-periodic oscillations and are thought to be caused by material moving along the inner edge of the accretion disk (the innermost stable circular orbit). As such their frequency is linked to the mass of the compact object. They can thus be used as an alternative way to determine the mass of candidate black holes.