Toward an Empirical Theory of Pulsar Emission XI. Understanding the Orientations of Pulsar Radiation and Supernova "Kicks"

TL;DR

This study analyzes pulsar polarization and proper motions to infer that supernova kicks are aligned with pulsar rotation axes, revealing that core emission is polarized perpendicular to the magnetic field and propagates as the extraordinary mode.

Contribution

It provides the first clear observational evidence linking pulsar proper motions to their rotation axes and clarifies the polarization mode of core emission in pulsars.

Findings

Proper motions are usually aligned with pulsar rotation axes.

Core emission polarization is perpendicular to the magnetic field.

Supernova kicks are likely parallel to the pulsar rotation axes.

Abstract

Two entwined problems have remained unresolved since pulsars were discovered nearly 50 years ago: the orientation of their polarized emission relative to the emitting magnetic field and the direction of putative supernova ``kicks' relative to their rotation axes. The rotational orientation of most pulsars can be inferred only from the (``fiducial') polarization angle of their radiation, when their beam points directly at the Earth and the emitting polar fluxtube field is $\parallel$ to the rotation axis. Earlier studies have been unrevealing owing to the admixture of different types of radiation (core and conal, two polarization modes), producing both $\parallel$ or $\perp$ alignments. In this paper we analyze the some 50 pulsars having three characteristics: core radiation beams, reliable absolute polarimetry, and accurate proper motions. The ``fiducial' polarization angle of the core emission, we then find, is usually oriented $\perp$ to the proper-motion direction on the sky. As the primary core emission is polarized $\perp$ to the projected magnetic field in Vela and other pulsars where X-ray imaging reveals the orientation, this shows that the proper motions usually lie $\parallel$ to the rotation axes on the sky. Two key physical consequences then follow: first, to the extent that supernova ``kicks' are responsible for pulsar proper motions, they are mostly $\parallel$ to the rotation axis; and second that most pulsar radiation is heavily processed by the magnetospheric plasma such that the lowest altitude ``parent' core emission is polarized $\perp$ to the emitting field, propagating as the extraordinary (X) mode.