Multimessenger Probes of the Supermassive Black Hole Binary Population: The Role of Pulsar Timing Arrays

TL;DR

This paper demonstrates that pulsar timing arrays can significantly improve constraints on supermassive black hole binary populations and their evolution, showcasing the potential of multimessenger astrophysics in understanding black hole demographics.

Contribution

It quantifies the extent to which PTA data can refine parameters of SMBHB populations using Bayesian inference on simulated datasets, highlighting the multimessenger approach's promise.

Findings

PTA data can reduce uncertainty in SMBHB hardening rate by over 50%.

PTA data can decrease the uncertainty in SMBHB lifetime by 25-75%.

There is an 88% chance PTA data will improve the characteristic mass variable constraints.

Abstract

By inferring the gravitational wave background (GWB) from a population of supermassive black hole binaries (SMBHBs), pulsar timing arrays (PTAs) enable the study of massive black holes. In many ways, PTAs manifest the promise of a multimessenger approach to astronomy: they can constrain SMBHB population characteristics that are otherwise difficult to constrain using electromagnetic observations, such as hardening mechanisms at sub-parsec separations. In this work, we quantify this multimessenger promise using Bayesian inference of many realizations of simulated PTA data, while adopting a model for the SMBHBs that has been successfully applied to the 15-year data set of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). Our analyses of 200 realistic, simulated NANOGrav data sets show that there is a greater than 50\% chance of reducing the prior uncertainty in the SMBHB hardening rate by more than 50\%, and in the SMBHB evolutionary lifetime by 25--75\%. Additionally, there is an 88\% chance that PTA data can reduce the prior uncertainty in the characteristic mass variable of the galaxy stellar mass function (GSMF) by 25--50\%. For $M_{\text{BH}}$--$M_{\text{Bulge}}$ parameters (in a model without redshift evolution) and the overall normalization parameter of the GSMF, PTA data can provide only marginal information gain beyond the constraints from electromagnetic observations. Our work delineates the domains over which electromagnetic and gravitational-wave data constrain the demographics and dynamics of the supermassive black-hole binary population, offering a clearer picture of the impact of population multi-messenger astrophysics probes with PTAs.