Pulsar timing array detections of supermassive binary black holes: implications from the detected common process signal and beyond

TL;DR

This paper explores how current pulsar timing array observations constrain supermassive binary black hole populations and predicts detection prospects for the gravitational wave background and individual black hole binaries with future telescopes.

Contribution

It introduces revised models of black hole populations considering recent PTA signals and estimates detection timelines for future observatories like CPTA and SKAPTA.

Findings

The MBH-host galaxy scaling relation needs significant evolution or higher normalization.

Detection probability for individual BBHs with current PTAs is very low.

Future PTAs could detect the GWB within 2-3 years, depending on the signal strength.

Abstract

Pulsar timing arrays (PTAs) are anticipated to detect the stochastic gravitational wave background (GWB) from supermassive binary black holes (BBHs) as well as the gravitational waves from individual BBHs. Recently, a common process signal was reported by several PTAs. In this paper, we investigate the constraints on the BBH population model(s) by current PTA observations and further study the detections of both the GWB and individual BBHs by current/future PTAs. We find that the MBH--host galaxy scaling relation, an important ingredient of the BBH population model, is required to either evolve significantly with redshift or have a normalization $\sim0.86-1.1$ dex higher than the empirical ones, if the GWB is the same as the common process signal. For both cases, the estimated detection probability for individual BBHs is too small for a positive detection by current PTAs. By involving either the constrained scaling relations or those empirical ones into the BBH population models, we estimate that the GWB may be detected with a signal-to-noise ratio $\gtrsim3$ by the PTAs based on the Five hundred meter Aperture Spherical radio Telescope (CPTA) and the Square Kilometer Array (SKAPTA) after $\sim2-3$ (or $\sim6-11$) years' observation, if it is the same as (or an order of magnitude lower than) the common process signal. The detection time of individual BBHs by CPTA and SKAPTA is close to that of the GWB detection. We show that the BBH population model can be strongly constrained by the number and property distributions of BBHs to be detected by future PTAs.