Limits on anisotropy in the nanohertz stochastic gravitational-wave background

TL;DR

This paper constrains the anisotropy of the nanohertz gravitational-wave background from supermassive black hole binaries using pulsar timing data, finding it consistent with isotropy and establishing new analysis methods.

Contribution

It presents the first constraints on the angular distribution of the nanohertz GWB from SMBHBs, introducing techniques for anisotropy analysis in pulsar timing data.

Findings

GWB is consistent with isotropy within 40% in higher multipoles.

New methods for analyzing anisotropy in gravitational-wave backgrounds.

Constraints derived from 2015 European Pulsar Timing Array data.

Abstract

The paucity of observed supermassive black hole binaries (SMBHBs) may imply that the gravitational wave background (GWB) from this population is anisotropic, rendering existing analyses sub-optimal. We present the first constraints on the angular distribution of a nanohertz stochastic GWB from circular, inspiral-driven SMBHBs using the $2015$ European Pulsar Timing Array data [Desvignes et al. (in prep.)]. Our analysis of the GWB in the $\sim 2 - 90$ nHz band shows consistency with isotropy, with the strain amplitude in $l>0$ spherical harmonic multipoles $\lesssim 40\%$ of the monopole value. We expect that these more general techniques will become standard tools to probe the angular distribution of source populations.