- Quasi-periodic oscillations
In high-energy (X-ray) astronomy, quasi-periodic oscillations (QPOs) refer to the way 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 acompact object such as awhite dwarf ,neutron star , orblack 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 ofgeneral 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 theLense-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 thetime series of the X-rays. A constant level ofwhite 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 (aDirac delta function given a long enough observation). A QPO, on the other hand, appears as a broader peak, sometimes with aLorentzian 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.
History
QPOs were first identified in white dwarf systems and then in neutron star systems. [http://adsabs.harvard.edu/abs/1985Natur.316..225V van der Klis et al. 1985, Nature, 316, 225] [http://adsabs.harvard.edu/abs/1986ApJ...306..230M Middleditch and Priedhorsky 1986, Astrophysical Journal 306, 230]
At first the neutron star systems found to have QPOs were of a class (
Z sources andatoll sources ) not known to have pulsations. The spin periods of these neutron stars were unknown as a result. These neutron stars are thought to have relatively low magnetic fields so the gas does not fall mostly onto their magnetic poles, as in accretingpulsar s. Because their magnetic fields are so low, the accretion disk can get very close to the neutron star before being disrupted by the magnetic field.The spectral variability of these neutron stars was seen to correspond to changes in the QPOs. Typical QPO frequencies were found to be between about 1 and 60
hertz . The fastest oscillations were found in a spectral state called the Horizontal Branch, and were thought to be a result of the combined rotation of the matter in the disk and the rotation of the collapsed star (the "beat frequency model"). During the Normal Branch and Flaring Branch, the star was thought to approach itsEddington luminosity at which the force of the radiation could repel the accreting gas. This could give rise to a completely different kind of oscillation.Observations starting in 1996 with the
Rossi X-ray Timing Explorer could detect faster variability, and it was found that neutron stars and black holes emit X-rays that have QPOs with frequencies up to 1000 hertz or so. Often "twin peak" QPOs were found in which two oscillations of roughly the same power appeared at high frequencies. These higher frequency QPOs may show behavior related to that of the lower frequency QPOs. [http://adsabs.harvard.edu/abs/2007ApJ...659L.145Y Wenfei Yu, 2007. Astrophysical Journal, 659, 145]Measuring Black Holes
QPOs can be used to determine the mass of
Black Holes . [cite web|url=http://www.eurekalert.org/pub_releases/2008-04/nsfc-nsi040108.php|title=NASA scientists identify smallest known black hole] The technique uses a relationship between black holes and the inner part of their surrounding disks, where gas spirals inward before reaching the event horizon. The hot gas piles up near the black hole and radiates a torrent of X-rays, with an intensity that varies in a pattern that repeats itself over a nearly regular interval. This signal is the QPO. Astronomers have long suspected that a QPO’s frequency depends on the black hole’s mass. The congestion zone lies close in for small black holes, so the QPO clock ticks quickly. As black holes increase in mass, the congestion zone is pushed farther out, so the QPO clock ticks slower and slower.References
ee also
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broad Iron K line
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