The Radio Polarisation of Quasars

Abstract

Observations over a wide range of wavelengths, 2.2 < lambda < 73 cm, have bee n combined to define the wavelength variation of the degree of linear polarisati on m(lambda) for 120 quasars with known redshift. For the majority, m(lambda) decreases monotonically with increasing wavelength but for 35 sources the polarisa tion curve is inverted at short wavelengths. A classification is given, based on both the polarisation curve and the radio spectrum, and the results are interpreted in terms of the presence or absence of opaque components in the source. The depolarisation which occurs at long wavelengths is accounted for by a combination of spectral effects and Faraday depolarisation. For 46 steep-spectrum sources the depolarisation curve appears to be dominated by the Faraday effect, and has been used to deduce the electron density within the radiating components. In this group of sources the correlation between depolarisation and redshift noted by Kronberg et al. is confirmed and strengthened. A discussion is given of some theoretical models of radio sources in the light of the depolarisation data.