Spectral-index asymmetries in radio galaxies: the effects of projection and light traveltime

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Abstract
Since the discovery of depolarization and spectral asymmetries between the two lobes of radio galaxies and of their correlation with jet-sidedness, there has been debate concerning the origin of these effects. For the majority of sources in which asymmetries are seen, they are in the sense that the lobe that is less depolarized also has the flatter spectrum and tends to be the lobe into which a one-sided jet, if present, is pointing. The correlation between jet-sidedness and depolarization asymmetry has been explained as a projection effect, in which the jet-side of the source is assumed to be the side nearer to us and the depolarization occurs in gas surrounding the source. The asymmetry in spectral index, on the other hand, has been explained either by invoking an asymmetric environment, or by assuming an intrinsically asymmetric or one-sided jet. In this paper we show that it is possible to obtain a very significant asymmetry in the spectral distribution from projection effects alone, provided that the magnetic flux density within the source decreases as the source evolves. The effect is sufficiently strong to produce a measurable effect even when the source axis is no more than 20° from the plane of the sky.

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