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The term quasi-periodic oscillation (QPO) is used in X-ray astronomy for a type of variability in X-ray sources that is interpreted as variability in the accretion of a compact object from its accretion disk. The variation is random, but can show a peak frequency (in the signal variation, not the EMR frequency) which can vary over time, with frequencies up to about 1000 Hz. Analysis can reveal information regarding the host and its accretion, one example being estimates of a central black hole's mass.