Equilibrium eccentricity of accreting binaries

TL;DR

This study uses high-resolution simulations to show that accreting binaries tend toward a specific eccentricity, with implications for gravitational wave observations and binary evolution understanding.

Contribution

It reveals the equilibrium eccentricity of accreting binaries and predicts observable signatures and eccentricity ranges for black hole binaries in gravitational wave detectors.

Findings

Equal-mass binaries evolve toward e ≈ 0.45 eccentricity.

Binaries with initial e ≲ 0.08 tend to circularize.

Predicted measurable eccentricities for black hole binaries in LISA band.

Abstract

Using high-resolution hydrodynamics simulations, we show that equal-mass binaries accreting from a circumbinary disk evolve toward an orbital eccentricity of $e \simeq 0.45$, unless they are initialized on a nearly circular orbit with $e \lesssim 0.08$, in which case they further circularize. The implied bi-modal eccentricity distribution resembles that seen in post-AGB stellar binaries. Large accretion spikes around periapse impart a tell-tale, quasi-periodic, bursty signature on the light curves of eccentric binaries. We predict that intermediate-mass and massive black hole binaries at $z \lesssim 10$ entering the \emph{LISA} band will have measurable eccentricities in the range $e \simeq 10^{-3} - 10^{-2}$, if they have experienced a gas-driven phase. On the other hand, GW190521 would have entered the LIGO/Virgo band with undetectable eccentricity $\sim 10^{-6}$ if it had been driven into the gravitational wave regime by a gas disk.