Orthogonal polarization mode phenomenon in pulsars

R. T. Gangadhara

Abstract:

We consider the polarization properties of radiation emitted by relativistic charged particles while moving along the curved magnetic field lines in a pulsar manetosphere. We propose that the radiation emitted by positrons and electrons while moving along the curved magnetic field lines is orthogonally polarized. The polarization angle of each orthogonal polarization mode is well described by the rotating vector model. However, the polarization angle swings observed in micropulses and subpulses are often found to be contrary to the rotating vector model, and such swings are expected to arise due to the coherent superposition of orthogonal polarization modes.

As an application of our model, we discuss the polarization of pulsar PSR B0950+08 at the frequency 1.71 GHz. Data on individual pulses, obtained using the 100-m Effelsberg radiotelescope, was statistically analysed and the results are presented as probability of occurrence gray-scale plots. We find that pulsars emit radiation mainly in the form of two independent orthogonal modes. It seems, they exist at all pulse longitudes but at some longitudes one mode dominates over the other. The polarization angle gray-plot indicates two most favoured angles separated by approximately 90 degree at each pulse longitude. The depolarization is mainly caused by the incoherent superposition of orthogonal modes.

We infer from this study that pulsar radiation consists of two major elliptically polarized electromagnetic waves with orthogonal polarization angles. Our model predicts that such waves could be radiated by the positrons and the electrons accelerated along the curved magnetic field lines. The sense of circular polarization of these modes depend upon from which side of particle trajectory the radiation is received.

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