The Gravitational-Wave Discovery Space of Pulsar Timing Arrays

TL;DR

This paper explores the potential of pulsar timing arrays to detect unexpected gravitational-wave sources, establishing constraints and highlighting the significance of GW memory effects at various frequencies and redshifts.

Contribution

It introduces a framework for assessing PTA discovery space for unanticipated sources and analyzes the detectability of GW memory and periodic signals.

Findings

Detection of GWs above ~3.e-5 Hz is highly unlikely without extraordinary coincidence.

GW memory can surpass direct GWs in detectability at high redshifts.

Periodic GWs in the 10^-8 to 10^-4.5 Hz band are distinguishable from pulsar parameter errors.

Abstract

Recent years have seen a burgeoning interest in using pulsar timing arrays (PTAs) as gravitational-wave (GW) detectors. To date, that interest has focused mainly on three particularly promising source types: supermassive--black-hole binaries, cosmic strings, and the stochastic background from early-Universe phase transitions. In this paper, by contrast, our aim is to investigate the PTA potential for discovering unanticipated sources. We derive significant constraints on the available discovery space based solely on energetic and statistical considerations: we show that a PTA detection of GWs at frequencies above ~3.e-5 Hz would either be an extraordinary coincidence or violate "cherished beliefs;" we show that for PTAs GW memory can be more detectable than direct GWs, and that, as we consider events at ever higher redshift, the memory effect increasingly dominates an event's total signal-to-noise ratio. The paper includes also a simple analysis of the effects of pulsar red noise in PTA searches, and a demonstration that the effects of periodic GWs in the 10^-8 -- 10^-4.5 Hz band would not be degenerate with small errors in standard pulsar parameters (except in a few narrow bands).