Circular polarization in radio pulsar PSR B1451−68: coherent mode transitions and intrabeam interference

Abstract

ABSTRACT The radio emission of pulsar B1451−68 contains two polarization modes of similar strength, which produce two clear orthogonal polarization angle tracks. When viewed on a Poincaré sphere, the emission is composed of two flux patches that rotate meridionally as a function of pulse longitude and pass through the Stokes V poles, which results in transitions between orthogonal polarization modes (OPMs). Moreover, the ratio of power in the patches is inversed once within the profile window. It is shown that the meridional circularization is caused by a coherent OPM transition (COMT) produced by a varying mode ratio at a fixed quarter-wave phase lag. The COMTs may be ubiquitous and difficult to detect in radio pulsar data, because they can leave no trace in polarized fractions and they are described by equation similar to the rotating vector model. The circularization, which coincides with flux minima at lower frequency, requires that profile components are formed by radiation with an oscillation phase that increases with longitude in steps of 90○ per component. The properties can be understood as an interference pattern involving two pairs of linear orthogonal modes (or two non-orthogonal elliptic waves). The frequency-dependent coherent superposition of coplanar oscillations can produce the minima in the pulse profile, and thereby the illusion of components as separate entities. The orthogonally polarized signal that is left after such negative interference explains the enhancement of polarization degree that is commonly observed in the minima between profile components.