Effects 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 rotation measure determinations of pulsars. Conventional analysis of high sensitivity polarimetric observations of PSR B2016+28 display variation of the rotation measure of $\pm$15 rad m$^{-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 polarisation 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.