Testing the Limits of Precision Pulsar Timing for Gravitational Wave Detection

Abstract

The NANOGrav collaboration and other pulsar timing arrays (PTAs) aim to use pulsar timing to detect low-frequency (nanohertz) gravitational waves. To do so, they have amassed sets of observations of millisecond pulsars spanning many years. Understanding the various processes that effect the time-of-arrival measurements made using these pulsar observations is extremely important for the goal of gravitational wave detection. Conversely, the pulsar observations are an extraordinarily rich source of information about pulsars and their astrophysical environments, including the interstellar medium along the line of sight to each pulsar, as well as gravitational waves. In this work, we investigate several such processes. Using the Gaia second data release, we measure the parallaxes and proper motions of several pulsars, allowing us to determine their distances and velocities within the Galaxy. We determine the form of the pulsar timing signals which would result from hyperbolic gravitational encounters between pulsars and interstellar objects, or from asteroid belts in binary pulsar systems, and outline methods for searching for signals of these forms. We examine in detail the effect of pulse shape changes on time-of-arrival measurements and develop methods for mitigating this effect. Finally, we describe observations of an recent, unexpectedly large pulse shape change affecting a PTA pulsar, and speculate as to its origins and consequences for pulsar timing.