Quasi-periodic oscillation

A quasi-periodic oscillation (QPO) is a distinctive X-ray signal following the outburst that comes from matter on the verge of falling into a black hole.

This tell-tale signal is a characteristic feature of the accretion disks that often surround the most compact objects in the universe — white dwarf stars, neutron stars and black holes. QPOs have been seen in many stellar-mass black holes, and there is tantalizing evidence for them in a few black holes that may have middleweight masses.

QPOs have also been detected around supermassive black holes — the type containing millions of stellar masses and located at the centers of galaxies. An example is the galaxy REJ 1034+396, which at a distance of 576 million light-years lies relatively nearby.