2015MNRAS.448..771W

The frequency dependence of pulsar linear polarization is investigated using simulations of emission and propagation processes. Linearly polarized waves are generated through curvature radiation by relativistic particles streaming along curved magnetic field lines, which have ordinary-mode (O-mode) and extraordinary-mode (X-mode) components. As the emitted waves propagate outwards, the two mode components are separated, due to refraction of the O mode, and their polarization states are also modified. According to the radius-to-frequency mapping, low-frequency emission is generated from the higher magnetosphere, where significant rotation effects lead the X and O modes to become separated. Hence, the low-frequency radiation has a large fraction of linear polarization. As the frequency increases, emission is generated from lower heights, where the rotation effect becomes weaker and the distribution regions of the two modes are more overlapped. Hence, more significant depolarization appears for emission at higher frequencies. In addition, the refraction effect of the O mode becomes significant in the very deep magnetosphere, which bends the O-mode emission towards the outer parts of a pulsar beam and also causes the separation of the mode distribution regions; hence the fractional linear polarization increases with frequency. If emission of different frequencies is generated from a region of the same height, significant O-mode refraction can result in the decrease of both profile width and fractional linear polarization. The observed frequency dependence of linear polarization for some pulsars can be explained naturally within the scope of our scenario.