Astrophysical constraints on massive black hole binary evolution from Pulsar Timing Arrays

TL;DR

This paper analyzes how current and future pulsar timing array data can constrain the population and evolution of massive black hole binaries, highlighting the limitations in inferring detailed properties without additional information.

Contribution

It demonstrates that PTA results set an upper limit on black hole merger density but cannot determine redshift or mass distribution without further assumptions.

Findings

Current PTAs limit black hole merger density independently of models

Future PTAs with increased sensitivity will only weakly constrain merger rates

Detection alone cannot precisely determine black hole merger properties without extra data

Abstract

We consider the information that can be derived about massive black-hole binary populations and their formation history solely from current and possible future pulsar timing array (PTA) results. We use models of the stochastic gravitational-wave background from circular massive black hole binaries with chirp mass in the range $10^6 - 10^{11} M_\odot$ evolving solely due to radiation reaction. Our parameterised models for the black hole merger history make only weak assumptions about the properties of the black holes merging over cosmic time. We show that current PTA results place an upper limit on the black hole merger density which does not depend on the choice of a particular merger history model, however they provide no information about the redshift or mass distribution. We show that even in the case of a detection resulting from a factor of 10 increase in amplitude sensitivity, PTAs will only put weak constraints on the source merger density as a function of mass, and will not provide any additional information on the redshift distribution. Without additional assumptions or information from other observations, a detection cannot meaningfully bound the massive black hole merger rate above zero or any particular mass.