Effect of quasi-orthogonal emission modes on the rotation measures of pulsars

We report here the discovery of a significant source of systematic error in the determinations of pulsar rotation measures. Conventional analysis of high-sensitivity polarimetric observations of PSR B2016+28 display variation in the rotation measure of +/-15 rad m(-2) (around the mean value of -34.6 rad m(-2)) across the pulse profile. Analysis of single pulse data shows that this variation is an artifact of the incoherent superposition of quasi-orthogonal polarization modes along with the frequency dependence of relative strength and/or quasi-orthogonality of the modes. Quasi-orthogonal polarization is common among pulsars, and therefore this effect needs to be taken into account in the interpretation of pulsar rotation measures.