Intermediate Polar - Light Curve Periodicities

Light Curve Periodicities

The light curve 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 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 of the light curve and producing a power spectrum. Intermediate polars produce spin and sideband periodicities in x-ray, ultraviolet, 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 typically have periods between 30 and 300 seconds.