Quasi-periodic oscillations
Encyclopedia
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.
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 pulsar
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 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. Wenfei Yu, 2007. Astrophysical Journal, 659, 145
X-ray astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...
, quasi-periodic oscillation (QPO) is the manner in which the X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
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
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
, neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
, or black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
.
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
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
's theory of general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
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
Time series
In statistics, signal processing, econometrics and mathematical finance, a time series is a sequence of data points, measured typically at successive times spaced at uniform time intervals. Examples of time series are the daily closing value of the Dow Jones index or the annual flow volume of the...
of the X-rays. A constant level of white noise
White noise
White noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...
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
Dirac delta function
The Dirac delta function, or δ function, is a generalized function depending on a real parameter such that it is zero for all values of the parameter except when the parameter is zero, and its integral over the parameter from −∞ to ∞ is equal to one. It was introduced by theoretical...
given a long enough observation). A QPO, on the other hand, appears as a broader peak, sometimes with a Lorentzian
Lorentzian
Lorentzian may refer to* Cauchy–Lorentz distribution, also known as the Lorentz distribution, Lorentzian function, or Cauchy distribution* Lorentz transformation* Lorentzian inner product* Lorentzian manifoldThe name Lorentz may refer to*Hendrik Lorentz...
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.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 pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
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
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
. 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
Eddington luminosity
The Eddington luminosity in a star is defined as the point where the gravitational force inwards equals the continuum radiation force outwards, assuming hydrostatic equilibrium and spherical symmetry. When exceeding the Eddington luminosity, a star would initiate a very intense continuum-driven...
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
Rossi X-ray Timing Explorer
The Rossi X-ray Timing Explorer is a satellite that observes the time structure of astronomical X-ray sources. The RXTE has three instruments—the Proportional Counter Array, the High-Energy X-ray Timing Experiment , and one instrument called the All Sky Monitor...
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. Wenfei Yu, 2007. Astrophysical Journal, 659, 145