On the Excess Dispersion in the Polarization Position Angle of Pulsar Radio Emission

Abstract

The polarization position angles (P.A.'s) of pulsar radio emission occupy a distribution that can be much wider than what is expected from the average linear polarization and the off-pulse instrumental noise. Contrary to our limited understanding of the emission mechanism, the excess dispersion in P.A. implies that pulsar P.A.'s vary in a random fashion. An eigenvalue analysis of the measured Stokes parameters is developed to determine the origin of the excess P.A. dispersion. The analysis is applied to sensitive, well-calibrated polarization observations of PSR B1929+10 and PSR B2020+28. The analysis clarifies the origin of polarization fluctuations in the emission and reveals that the excess P.A. dispersion is caused by the isotropic inflation of the data-point cluster formed from the measured Stokes parameters. The inflation of the cluster is not consistent with random fluctuations in P.A., as might be expected from random changes in the orientation of the magnetic field lines in the emission region or from stochastic Faraday rotation in either the pulsar magnetosphere or the interstellar medium. The inflation of the cluster, and thus the excess P.A. dispersion, is attributed to randomly polarized radiation in the received pulsar signal. The analysis also indicates that orthogonal polarization modes (OPMs) occur where the radio emission is heavily modulated. In fact, OPMs may occur only where the modulation index exceeds some critical value, βc 0.3.