Systematic and Stochastic Variations in Pulsar Dispersion Measures

TL;DR

This paper investigates both deterministic and stochastic variations in pulsar dispersion measures caused by interstellar medium dynamics, pulsar motion, and solar system effects, providing a comprehensive model for understanding DM fluctuations in pulsar timing.

Contribution

It introduces a unified framework accounting for line-of-sight and transverse pulsar motions, electron-density variations, and solar influences to explain DM variations and structure functions.

Findings

Pulsar motions parallel to the line of sight can explain linear DM trends.

Interstellar medium gradients and pulsar velocities contribute significantly to DM variance.

Periodic DM changes can result from solar, heliospheric, and ionospheric effects.

Abstract

We analyze deterministic and random temporal variations in dispersion measure (DM) from the full three-dimensional velocities of pulsars with respect to the solar system, combined with electron-density variations on a wide range of length scales. Previous treatments have largely ignored the pulsar's changing distance while favoring interpretations involving the change in sky position from transverse motion. Linear trends in pulsar DMs seen over 5-10~year timescales may signify sizable DM gradients in the interstellar medium (ISM) sampled by the changing direction of the line of sight to the pulsar. We show that motions parallel to the line of sight can also account for linear trends, for the apparent excess of DM variance over that extrapolated from scintillation measurements, and for the apparent non-Kolmogorov scalings of DM structure functions inferred in some cases. Pulsar motions through atomic gas may produce bow-shock ionized gas that also contributes to DM variations. We discuss possible causes of periodic or quasi-periodic changes in DM, including seasonal changes in the ionosphere, annual variation of the solar elongation angle, structure in the heliosphere-ISM boundary, and substructure in the ISM. We assess the solar cycle's role on the amplitude of ionospheric and solar-wind variations. Interstellar refraction can produce cyclic timing variations from the error in transforming arrival times to the solar system barycenter. We apply our methods to DM time series and DM gradient measurements in the literature and assess consistency with a Kolmogorov medium. Finally, we discuss the implications of DM modeling in precision pulsar timing experiments.