On The Depolarization Asymmetry Seen in Giant Radio Lobes

TL;DR

This paper investigates the depolarization asymmetry in giant radio lobes, analyzing whether it is caused by internal lobe differences or external Faraday screens, and concludes that internal differences are likely responsible in certain FRI sources.

Contribution

The study provides evidence that the Laing-Garrington effect in some FRI radio sources is due to internal lobe differences rather than external Faraday screens, challenging previous assumptions.

Findings

Depolarization asymmetry in FRI sources is likely due to internal lobe differences.

Jet one-sidedness in these sources cannot be explained by relativistic beaming.

The Laing-Garrington effect may have a common origin in both FRI and FRII sources.

Abstract

The depolarization asymmetry seen in double-lobed radio sources, referred to as the Laing-Garrington (L-G) effect where more rapid depolarization is seen in the lobe with no visible jet as the wavelength increases, can be explained either by internal differences between the two lobes, or by an external Faraday screen that lies in front of only the depolarized lobe. If the jet one-sidedness is due to relativistic beaming the depolarization asymmetry must be due to an intervening Faraday screen. If it is intrinsic the depolarization asymmetry must be related to internal differences in the lobes. We assume in this paper that the speed in the outer jet of several Fanaroff-Riley Class 1 (FRI) sources exhibiting the L-G effect is close to the 0.1c reported by several other investigators. For these sources we find that the jet one-sidedness cannot be explained by beaming and therefore must be intrinsic. In these FRI sources the L-G effect must be due to differences that originate inside the lobes themselves. Although it is not known if the flow in the outer jets of FRII sources also slows to this speed it is suggested that the explanation of the L-G effect is likely to be the same in both types. This argument is strengthened by the recent evidence that FRII galaxies have very large viewing angles, which in turn implies that the L-G model cannot work regardless of the jet velocity. It may therefore be too soon to completely rule out internal depolarization in the lobes as the true explanation for the L-G effect.