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

Abstract By inferring the gravitational wave background 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 subparsec 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 yr dataset of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). Our analyses of 200 realistic, simulated NANOGrav datasets 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 BH – M 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 multimessenger astrophysics probes with PTAs.