Low-frequency polarimetry of the millisecond pulsar PSR1937 + 214

Abstract

Pulsars with periods of a few milliseconds^1,2 test theories of the radio emission process in extreme conditions. The velocity of light radius, which is an important scale-length in most theories, is less than 10 stellar radii for the millisecond pulsar PSR1937 + 214, and the inferred surface magnetic field is a few times 10⁸ G, less than 10⁻³ that inferred for the Crab pulsar. Polarization observations across both the main pulse and the prominent interpulse can help clarify the emission geometry and probe similarities between this pulsar and others, such as the Crab pulsar, that have very different magnetospheric conditions. High-frequency polarization observations exist for this pulsar^3,4, but the temporal smearing produced by interstellar dispersion has hampered low-frequency observations. We now report polarization observations at 431 MHz that have 4 µs time resolution and are free from the effects of interstellar dispersion. We find that the interpulse polarization decreases towards lower frequencies, opposite in trend to the polarization of the main pulse and of most other pulsars. The position angles at the centre of the main pulse and interpulse are approximately equal and the position angle gradients have the same sign, supporting a model in which the main pulse and interpulse are emitted above opposite magnetic poles.