23 - Polarization characteristics of rotation-powered pulsars

Summary

Polarization measurements of rotation-powered pulsars have been a very powerful diagnostic in the radio band and promise to be at least as useful a diagnostic in the X-ray band. Since the relativistic particles that radiate pulsar high-energy emission tightly beam the radiation along the pulsar magnetic field, phase-resolved polarimetry has the potential to map the emission patterns. Fermi observations of young pulsars at gamma-ray energies have disfavored polar-cap models where emission takes place near the neutron star surface, and strongly favor outer-magnetosphere models where emission takes place close to the light cylinder or beyond. Since the different outer-magnetosphere models predict similar gamma-ray light curves, it is difficult to discriminate between them at gamma-ray energies. But X-ray polarization has the potential to provide this discrimination, since the models predict distinct polarization signatures and optical detections will only be possible for a small number of pulsars.

Introduction

Rotating systems are particularly interesting to study with polarization because the rotation provides changing views of the emitting regions. Rotation-powered pulsars have the additional advantage that the emission is radiated by highly relativistic particles moving along magnetic field lines and therefore the emission angle measures the field direction to within an angle of 1/γ, where γ is the particle Lorentz factor. The magnetic field structure is also able to be derived independently, using either retarded-vacuum, force-free (e.g.) or pair-starved solutions.