Rotational asymmetry of pulsar profiles

TL;DR

This paper investigates how rotation affects the shape of pulse profiles in millisecond pulsars, revealing a caustic phase where emission is maximized and polarization changes are most pronounced, with implications for emission mechanisms.

Contribution

It introduces a unified analysis of rotational effects on pulsar emission by considering curvature of electron trajectories in the inertial observer's frame, identifying a caustic phase with distinctive observational signatures.

Findings

Identification of a caustic phase with maximum emission and polarization gradient.

Demonstration of asymmetry effects on curvature radiation and pair production.

Application to the 5 ms pulsar J1012+5307 confirming theoretical predictions.

Abstract

We analyse the influence of rotation on shapes of pulse profiles of fast-rotating (millisecond) pulsars. Corotation has two opposing effects: 1) the caustic enhancement of the trailing side (TS) by aberration and retardation (AR), which squeezes the emission into a narrower phase interval; 2) the weakening of the TS caused by the asymmetry of curvature radiation about the dipole axis. Analysis of the radii of curvature of electron trajectories in the inertial observer's frame (IOF) enables these two effects to be considered together. We demonstrate that for dipolar magnetic field lines on the TS there exists a `caustic phase' beyond which no emission can be observed. This phase corresponds to the zero (or minimum) curvature of the IOF trajectories and maximum bunching of the emission. The maximum gradient of polarisation angle (PA) in the S-shaped PA curve is also associated with the curvature minimum and occurs at exactly the same phase. The asymmetry of trajectory curvature with respect to the dipole axis affects the curvature emissivity and the efficiency of pair production, suggesting a minimum at the caustic phase. Emission over a fixed range of altitudes, as expected in millisecond pulsars, leads to broad leading profiles and sharp peaks with a cutoff phase on the TS. We apply our results to the main pulse of the 5 ms pulsar J1012+5307.