Rapid Rotation of Polarization Orientations in PSR B1919+21's Single Pulses: Implications On Pulsar's Magnetospheric Dynamics

TL;DR

This study investigates rapid polarization angle rotations in single pulses of PSR B1919+21, revealing their association with magnetospheric plasma dynamics and proposing a model to explain these phenomena.

Contribution

The paper introduces a phenomenological model linking polarization rotations to phase lag changes in a pulsar's magnetosphere, supported by observational data and dispersion relation analysis.

Findings

Over one-third of pulses show PA rotations over 180 degrees.

PA rotations are often accompanied by oscillations in circular polarization.

The emission height is inferred to be close to 10 neutron star radii.

Abstract

We analyze and model rapid rotations of polarization orientations in PSR B1919+21's single pulses based on Five-hundred-meter Aperture Spherical radio Telescope observation data. In more than one-third of B1919+21's single pulses, the polarization position angle (PA) is found to rotate quasi-monotonically with pulse longitude, by over 180 degrees or even 360 degrees. Some single pulse PA even rotates by over 540 degrees. Most of these quasi-monotonic PA curves have negative slopes with respect to pulse longitude. Oscillations of circular polarization fraction accompany these PA rotations. This rapid rotation could be induced by a quick change of phase lag between two normal wave modes within an individual pulse. We propose a phenomenological model to reproduce the observed polarization rotations in single pulses, and calculate phase lags in a dipolar magnetic field of an aligned rotating pulsar, with a dispersion relation of orthogonal wave modes in strongly magnetized electron-positron plasma. According to the dispersion relation, the weak frequency dependence of observed polarization rotation requires small angles between the radio wavevector and local magnetic fields, which requires the radio emission height to be low, on the order of 10 times neutron star radius.