No tension between pulsar timing array upper limits on the nano-Hertz gravitational wave background and assembly models of massive black hole binaries

Hannah Middleton,Alberto Sesana,Siyuan Chen,Walter Del Pozzo,Alberto Vecchio

doi:10.1088/1742-6596/1468/1/012214

Abstract

Pulsar timing arrays provide a means to observe the nano-Hertz gravitational wave background from the population of merging massive black hole binaries. Observations are placing increasingly stringent upper limits on the gravitational wave background. Upper limits and future detections will enable the study of the properties of the merging population. Recent upper limits have cast doubt on current predictions of the gravitational wave background. Here we perform a Bayesian analysis comparing upper limits to astrophysical prediction. So far models are consistent with observation. These proceedings summarise previous work in Ref. [1].

Highlights

Hierarchical formation scenarios point towards frequent mergers of galaxies throughout cosmic time
We see that in all cases, the top of the prior distribution is removed by the upper limit, all astrophysical predictions remain consistent with the upper limit
Upper limits are beginning to reach sensitivities where meaningful comparisons can be made with astrophysical predictions of the MBHB population

Summary

Introduction

Hierarchical formation scenarios point towards frequent mergers of galaxies throughout cosmic time. It is likely that the evolution of the central massive black holes (MBH) within merging galaxies goes hand-in-hand with galaxy evolution, producing a population of merging MBH binaries (MBHBs). Gravitational wave (GW) searches at nano-Hertz (nHz) frequencies will provide insight into the properties of this population. Many merging binaries produce a stochastic GW background (GWB). Timing a selection of ultra-stable millisecond pulsars creates a galactic-scale GW detector [5]. Pulsar timing array (PTA) campaigns around the world are hunting for the nHz GWB. No detection has been made so far, the three PTA consortia have been progressively placing more constraining upper limits on the GWB

Objectives

Results

Conclusion