Intermediate polar
Encyclopedia
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
system. In most cataclysmic variables, matter from a main-sequence companion star is gravitationally stripped by 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...
star in the form of an accretion disk. In intermediate polar systems, the accretion disk is disrupted by the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
of its white dwarf star. Infalling gas from the companion star will form a disk when the matter is still far from the white dwarf, but will follow magnetic field lines in accretion streams when the matter falls closer to the white dwarf. In the transition region between the accretion disk and the accretion stream, infalling gas may also fall as curved sheets called accretion curtains.
The name "intermediate polar" is derived from the strength of the white dwarf's magnetic field, which is between that of non-magnetic cataclysmic variable systems and strongly-magnetic systems. Non-magnetic systems exhibit full accretion disks, while strongly magnetic systems (called polar
Polar (cataclysmic variable)
A Polar is a type of cataclysmic variable binary star system with a very strong magnetic field.In most cataclysmic variables, matter from a main-sequence companion star is gravitationally stripped by a white dwarf star in the form of an accretion disk. In polar systems, the magnetic field of the...
s, or AM Herculis
AM Herculis
AM Herculis is a red dwarf variable star located in the constellation Hercules. This star, along with the star AN Ursae Majoris, is the prototype for a category of cataclysmic variable stars called polars, or AM Her type stars.-History:...
systems) exhibit only accretion streams.
There are 26 confirmed intermediate polar systems as of 14 April 2006. This represents about 1% of the 1,830 total cataclysmic variable systems presented by Downes et al. (2006) in the Catalog of Cataclysmic Variables.
Physical Properties
Intermediate polar systems are strong x-rayX-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...
emitters. The x-rays are generated by high velocity particles from the accretion stream forming a shock
Shock (mechanics)
A mechanical or physical shock is a sudden acceleration or deceleration caused, for example, by impact, drop, kick, earthquake, or explosion. Shock is a transient physical excitation....
as they fall onto the surface of the white dwarf star. As particles decelerate and cool before hitting the white dwarf surface, bremsstrahlung
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
x-rays are produced and may subsequently be absorbed by gas surrounding the shock region.
The magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
strength of white dwarfs in intermediate polar systems are typically 1 million to 10 million gauss (100–1000 tesla
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
s). This is about a million times stronger than the Earth's magnetic field
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
and towards the upper limit of magnetic field strengths that can be produced in a laboratory on Earth, but is much less than the magnetic field strength of 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...
s. At the intersection of the accretion stream and the surface of the white dwarf, a hot spot is produced. Because the white dwarf has a dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
magnetic field, it will have one hot spot at each of its magnetic poles. As the white dwarf and its dipole magnetic field spin, the hot spots will spin also.
Other defining characteristics of intermediate polars include a strong Helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
II emission line
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
at 468.1 nm and circular polarization
Circular polarization
In electrodynamics, circular polarization of an electromagnetic wave is a polarization in which the electric field of the passing wave does not change strength but only changes direction in a rotary type manner....
, in addition to the light curve periodicities described below.
Light curve periodicities
The light curveLight curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band...
of an intermediate polar may show several types of stable periodic changes in brightness. One periodicity is related to the orbital period of the binary star system. The orbital periods of confirmed intermediate polars range from 1.4 to 48 hours, with typical values between 3 and 6 hours.
A second periodic signal originates from the rotation of the white dwarf spinning on its axis. The observational characteristic that most clearly defines an intermediate polar is the existence of a spin period signal that is shorter than the orbital period. The known periods range from 33 to 4022 seconds. Explanations for the physical cause of the spin period oscillations involve the white dwarf hot spot and/or the converging material above the hot spot.
A third light curve periodicity, the sideband
Sideband
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing power as a result of the modulation process. The sidebands consist of all the Fourier components of the modulated signal except the carrier...
period between the spin period and the orbital period, is also often present.
All three periodic signals may be measured by taking a fourier transform
Fourier transform
In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...
of the light curve and producing a power spectrum. Intermediate polars produce spin and sideband periodicities in x-ray, ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
, and optical wavelengths. Although the source of the periods in all three wavelengths is ultimately the white dwarf spin, the exact mechanisms for producing the high-energy periodicities and the optical periodicities are thought to be different.
In addition to the stable oscillations, unstable oscillations called "quasi-periodic oscillations" may appear and then die off after a few cycles. Quasi-periodic oscillations
Quasi-periodic oscillations
In X-ray astronomy, quasi-periodic oscillation is the manner in which the X-ray light from an astronomical object flickers about certain frequencies...
typically have periods between 30 and 300 seconds.