Effelsberg 100-m polarimetric observations of a sample of Compact Steep-Spectrum sources

TL;DR

This study presents polarimetric measurements of Compact Steep-Spectrum sources using the Effelsberg 100-m telescope, revealing frequency-dependent polarization behavior, rotation measures, and challenging previous correlations with source size.

Contribution

First comprehensive polarimetric analysis of CSS sources at multiple frequencies, highlighting the influence of foreground screens and questioning the 'Cotton effect' due to sample contamination.

Findings

Depolarisation occurs below 2 GHz.

Rotation measures range from -20 to 3900 rad/m².

Repolarisation and variability observed in some sources.

Abstract

We completed observations with the Effelsberg 100-m radio telescope to measure the polarised emission from a complete sample of Compact Steep-Spectrum sources. We observed the sources at four different frequencies. We complemented these measurements with polarisation parameters at 1.4 GHz derived from the NVSS. Previous single dish measurements were taken from the catalogue of Tabara and Inoue. The depolarisation index DP was computed for four pairs of frequencies. A drop in the fractional polarisation appeared in the radio emission when observing at frequencies below about 2 GHz. Rotation measures were derived for about 25% of the sources in the sample. The values range from about -20 rad/m**2} found for 3C138 to 3900 rad/m**2 in 3C119. In all cases, the lambda**2 law is closely followed. The presence of a foreground screen as predicted by the Tribble model or with ``partial coverage'' as defined by ourselves can explain the polarimetric behaviour of the CSS sources detected in polarisation by the present observations. Indication of repolarisation at lower frequencies was found for some sources. A case of possible variability in the fractional polarisation is also suggested. The most unexpected result was found for the distribution of the fractional polarisations versus the linear sizes of the sources. Our results appear to disagree with the findings of Cotton and collaborators and Fanti and collaborators for the B3-VLA sample of CSS sources, the so-called ``Cotton effect''. This apparent contradiction may, however, be caused by the large contamination of the sample by quasars with respect to the B3-VLA.