2013MNRAS.435..311O

We present full Stokes radio polarization observations of the quasar PKS B2126-158 (z = 3.268) from 1 to 10GHz using the Australia Telescope Compact Array. The source has large fractional circular polarization (CP), mc ≡ {verbar}V{verbar}/I, detected at high significance across the entire band (from 15 to 90σ per 128MHz subband). This allows us to construct the most robust CP spectrum of an active galactic nucleus (AGN) jet to date. We find mc ∝ ν+0.60±0.03 from 1.5 to 6.5GHz, with a peak of mc ∼ 1 percent before the spectrum turns over somewhere between 6.5 and 8GHz, above which mc ∝ ν-3.0±0.4. The fractional linear polarization (LP; p) varies from ≲ 0.2 to ∼ 1 percent across our frequency range and is strongly anticorrelated with the fractional CP, with a best-fitting power law giving mc ∝ p-0.24±0.03. This is the first clear relation between the observed LP and CP of an AGN jet, revealing the action of Faraday conversion of LP to CP within the jet. More detailed modelling in conjunction with high spatial resolution observations are required to determine the true driving force behind the conversion (i.e. magnetic twist or internal Faraday rotation). In particular determining whether the observed Faraday rotation is internal or entirely external to the jet is key to this goal. The simplest interpretation of our observations favours some internal Faraday rotation, implying that Faraday rotation-driven conversion of LP to CP is the dominant CP generation mechanism. In this case, a small amount of vector-ordered magnetic field along the jet axis is required, along with internal Faraday rotation from the low-energy end of the relativistic electron energy spectrum in an electron-proton-dominated jet.