Quasi-periodic Oscillation

In X-ray astronomy, quasi-periodic oscillation (QPO) is the manner in which the X-ray light from an astronomical object flickers about certain frequencies. In these situations, the X-rays are emitted near the inner edge of an accretion disk in which gas swirls onto a compact object such as a white dwarf, neutron star, or black hole.

The QPO phenomenon promises to help astronomers understand the innermost regions of accretion disks and the masses, radii, and spin periods of white dwarfs, neutron stars, and black holes. QPOs could help test Albert Einstein's theory of general relativity which makes predictions that differ most from those of Newtonian gravity when the gravitational force is strongest or when rotation is fastest (when a phenomenon called the Lense-Thirring effect comes into play.) However, the various explanations of QPOs remain controversial and the conclusions reached from their study remain provisional.

A QPO is identified by performing a power spectrum of the time series of the X-rays. A constant level of white noise is expected from the random variation of sampling the object's light. Systems that show QPOs sometimes also show nonperiodic noise that appears as a continuous curve in the power spectrum. A periodic pulsation appears in the power spectrum as a peak of power at exactly one frequency (a Dirac delta function given a long enough observation). A QPO, on the other hand, appears as a broader peak, sometimes with a Lorentzian shape.

What sort of variation with time could cause a QPO? For example, the power spectrum of an oscillating shot appears as a continuum of noise together with a QPO. An oscillating shot is a sinusoidal variation that starts suddenly and decays exponentially. A scenario in which oscillating shots cause the observed QPOs could involve "blobs" of gas in orbit around a rotating, weakly magnetized neutron star. Each time a blob comes near a magnetic pole, more gas accretes and the X-rays increase. At the same time, the blob's mass decreases so that the oscillation decays.

Often power spectra are formed from several time intervals and then added together before the QPO can be seen to be statistically significant.