Polarimetry of the millisecond pulsar

Abstract

The millisecond pulsar¹ PSR1937 + 21 might be expected to have unique waveform and polarization properties as a result of its remarkably short period and small spin-down rate^2,3. The proximity of the velocity of light cylinder to the star (R_LC = 7.4 × 10⁶ cm) and the low value of inferred magnetic field (∼10⁸ G, 10⁻⁴ that of the Crab pulsar) should strongly influence the magnetospheric structure and particle density of this pulsar. And yet the waveform is similar to that of the Crab pulsar⁴ and PSR1055 − 52 (ref. 5), which have interpulses that are comparable in strength to the main pulse, and the polarization properties are at first sight unexceptional². I present here polarization observations of the main pulse and interpulse at 1,415 and 2,380 MHz that have higher time resolution than previously available. The main pulse depolarizes substantially over this frequency range whereas the interpulse polarization almost doubles. There is evidence that orthogonally polarized radiation is present on the leading edge of the main pulse and the trailing edge of the interpulse and that it accounts for the ∼90° difference in position angle between the main pulse and interpulse. The interpulse decreases in intensity relative to the main pulse as f⁻¹, indicating (if the trend continues to lower frequency) that the high frequency interpulse dominates the main pulse below ∼700 MHz. The main pulse is found to consist of two closely spaced components.