History
QPOs were first identified in white dwarf systems and then in neutron star systems.
At first the neutron star systems found to have QPOs were of a class (Z sources and atoll 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 accreting pulsars. 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 its Eddington 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 amplitutes. These higher frequency QPOs may show behavior related to that of the lower frequency QPOs.
Read more about this topic: Quasi-periodic Oscillation
Famous quotes containing the word history:
“So in accepting the leading of the sentiments, it is not what we believe concerning the immortality of the soul, or the like, but the universal impulse to believe, that is the material circumstance, and is the principal fact in this history of the globe.”
—Ralph Waldo Emerson (1803–1882)
“I think that Richard Nixon will go down in history as a true folk hero, who struck a vital blow to the whole diseased concept of the revered image and gave the American virtue of irreverence and skepticism back to the people.”
—William Burroughs (b. 1914)
“The history of all Magazines shows plainly that those which have attained celebrity were indebted for it to articles similar in natureto Berenice—although, I grant you, far superior in style and execution. I say similar in nature. You ask me in what does this nature consist? In the ludicrous heightened into the grotesque: the fearful coloured into the horrible: the witty exaggerated into the burlesque: the singular wrought out into the strange and mystical.”
—Edgar Allan Poe (1809–1849)