Published online by Cambridge University Press: 05 June 2012
Sources of polarized radiation
Polarization of electromagnetic radiation may occur in a variety of astrophysical circumstances. In some cases the polarization may be considered to be intrinsic to the source (arising simultaneously with the radiation). One example is synchrotron radiation, in which the radiation arises from energetic electrons spiraling along magnetic field lines. The polarization of synchrotron radiation will be in general elliptical and will depend on the angle between the line of sight and the magnetic field. See Figure 17.1 for geometries giving rise to linear, elliptical, and circular polarization. For relativistic electrons, radiation is beamed in the forward direction, making linear polarization more likely to be seen than circular polarization. Linear polarizations approaching 50% are common at centimeter wavelengths in extragalactic radio jets. For a synchrotron spectral index of -1 (I(ν) ∝ ν-1), linear polarization up to 75% is possible (Saikia & Salter, 1988).
The Zeeman effect also produces polarized radiation. For a magnetic quantum numberM, magnetic fields split energy levels into 2M+1 magnetic sublevels. Radiation will then be linearly or circularly polarized depending on the direction of radiation with respect to the magnetic field and the change in the projected magnetic quantum number, as indicated in Figure 17.2. Relevant magnetic fields vary from 10-6 gauss for the interstellar medium, to 10 000 gauss for magnetic stars. Compact objects such as white dwarfs can have even higher fields.