Equilibrium Eccentricity of Accreting Binaries

Using high-resolution hydrodynamics simulations, we show that equal-mass binaries accreting from a circumbinary disk evolve toward an orbital eccentricity of e 0.45, unless they are initialized on a nearly circular orbit with e less than or similar to 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, quasiperiodic, bursty signature on the light curves of eccentric binaries. We predict that intermediate-mass and massive black hole binaries at z less than or similar to 10 entering the LISA band will have measurable eccentricities in the range of e 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 similar to 10(-6) if it had been driven into the gravitational-wave regime by a gas disk.

Automated summary

High-resolution hydrodynamics simulations show that equal-mass binaries accreting from a circumbinary disk will evolve towards a specific eccentricity orbit, with large accretion spikes leaving a distinct signature on light curves. Intermediate-mass and massive black hole binaries entering the LISA band will have measurable eccentricities, while GW190521 would have undetectable eccentricity entering the LIGO/Virgo band if driven by a gas disk into the gravitational-wave regime.