Broad-band radio circular polarization spectrum of the relativistic jet in PKS B2126-158

We present full Stokes radio polarization observations of the quasar PKS B2126-158 (z = 3.268) from 1 to 10 GHz using the Australia Telescope Compact Array. The source has large fractional circular polarization (CP), m_c = |V|/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 m_c αν^{+0.60 ± 0.03} from 1.5 to 6.5 GHz, with a peak of mc ~ 1 per cent before the spectrum turns over somewhere between 6.5 and 8 GHz, above which m_c αν^{-3.0 ± 0.4}. The fractional linear polarization (LP; p) varies from <0.2 to ~1 per cent across our frequency range and is strongly anticorrelated with the fractional CP, with a best-fitting power law giving m_c α 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.