Dissecting the nanoHz gravitational wave sky: frequency-correlated anisotropy induced by eccentric supermassive black hole binaries

TL;DR

This paper proposes a method to identify eccentric supermassive black hole binaries in nanoHz gravitational wave data by analyzing frequency-correlated anisotropy in sky maps, helping distinguish their signals from circular binaries.

Contribution

It introduces a novel approach using sky map correlations across frequencies to detect eccentric SMBHBs, breaking the degeneracy caused by environmental effects.

Findings

Eccentric SMBHBs produce broadband sky maps similar across frequencies.

The method can detect eccentricity at 3σ in over 50% of simulated cases.

Sky map correlation analysis effectively distinguishes eccentric from circular binaries.

Abstract

Revealing the nature of the nanoHz gravitational wave (GW) signal recently reported by Pulsar Timing Arrays (PTAs) collaborations around the world is the next goal of low-frequency GW astronomy. The signal likely originates from the incoherent superposition of GWs emitted by a cosmological population of supermassive black hole binaries (SMBHBs). Those binaries can be highly eccentric and/or strongly coupled to their nuclear environment, resulting in an attenuation of the overall GW signal at low frequencies. In this paper, we propose to use the correlation properties of the distributed GW power in the sky across the frequency spectrum as a smoking gun for eccentric SMBHBs thus allowing to break the spectral degeneracy between eccentricity and environmental effect. The simple underlying idea is that, contrary to circular binaries, eccentric ones emit a broadband spectrum thus resulting in similar sky maps at different frequencies. We first demonstrate the applicability of this simple concept on sky maps constructed directly from the theoretical sky distribution of the GWB power induced by realistic populations of SMBHBs. We then demonstrate the viability of this analysis on simulated SKA-like PTA data. By statistically comparing sky maps reconstructed from hundreds injected circular and highly eccentric SMBHB populations, we find that eccentricity can be detected at $3\sigma$ in more than $50\%$ of cases.