Gravitational waves from resolvable massive black hole binary systems and observations with Pulsar Timing Arrays

TL;DR

This paper explores the potential of Pulsar Timing Arrays, especially the SKA, to detect gravitational waves from resolvable massive black hole binaries, highlighting their role in complementing future space-based observations.

Contribution

It assesses the expected number and characteristics of resolvable massive black hole binary sources detectable by PTAs, including the SKA, across various population models.

Findings

At least one resolvable source may produce timing residuals of 5-50 ns.

PTAs can detect these sources despite their rarity.

Observations will complement LISA's high-mass black hole surveys.

Abstract

Massive black holes are key components of the assembly and evolution of cosmic structures and a number of surveys are currently on-going or planned to probe the demographics of these objects and to gain insight into the relevant physical processes. Pulsar Timing Arrays (PTAs) currently provide the only means to observe gravitational radiation from massive black hole binary systems with masses >10^7 solar masses. The whole cosmic population produces a stochastic background that could be detectable with upcoming Pulsar Timing Arrays. Sources sufficiently close and/or massive generate gravitational radiation that significantly exceeds the level of the background and could be individually resolved. We consider a wide range of massive black hole binary assembly scenarios, we investigate the distribution of the main physical parameters of the sources, such as masses and redshift, and explore the consequences for Pulsar Timing Arrays observations. Depending on the specific massive black hole population model, we estimate that on average at least one resolvable source produces timing residuals in the range ~5-50 ns. Pulsar Timing Arrays, and in particular the future Square Kilometre Array (SKA), can plausibly detect these unique systems, although the events are likely to be rare. These observations would naturally complement on the high-mass end of the massive black hole distribution function future surveys carried out by the Laser Interferometer Space Antenna (LISA)