The eccentricity distribution of compact binaries

TL;DR

This paper investigates the expected eccentricity distributions of compact binary inspirals at frequencies relevant to future gravitational wave detectors, revealing most binaries will have very low eccentricities, with some NS-NS binaries potentially exhibiting significant eccentricity.

Contribution

It provides the first detailed analysis of eccentricity distributions at multiple detector frequencies using population synthesis models, highlighting the rarity of high-eccentricity inspirals.

Findings

Most BH-BH and BH-NS binaries have negligible eccentricities at detector entry.

A small but notable fraction of NS-NS binaries may have eccentricities above 0.01.

Eccentricity fractions increase with lower detection frequencies, especially for NS-NS binaries.

Abstract

The current gravitational wave detectors have reached their operational sensitivity and are nearing detection of compact object binaries. In the coming years, we expect that the Advanced LIGO/VIRGO will start taking data. At the same time, there are plans for third generation ground-based detectors such as the Einstein Telescope, and space detectors such as DECIGO. We discuss the eccentricity distribution of inspiral compact object binaries during they inspiral phase. We analyze the expected distributions of eccentricities at three frequencies that are characteristic of three future detectors: Advanced LIGO/VIRGO (30 Hz), Einstein Telescope (3 Hz), and DECIGO (0.3 Hz). We use the StarTrack binary population code to investigate the properties of the population of compact binaries in formation. We evolve their orbits until the point that they enter a given detector sensitivity window and analyze the eccentricity distribution at that time. We find that the eccentricities of BH-BH and BH-NS binaries are quite small when entering the Advanced LIGO/VIRGO detector window for all considered models of binary evolution. Even in the case of the DECIGO detector, the typical eccentricities of BH-BH binaries are below 10^{-4}, and the BH-NS eccentricities are smaller than 10^{-3}. Some fraction of NS-NS binaries may have significant eccentricities. Within the range of considered models, we found that a fraction of between 0.2% and 2% NS-NS binaries will have an eccentricity above 0.01 for the Advanced LIGO/VIRGO detectors. For the ET detector, this fraction is between 0.4% and 4%, and for the DECIGO detector it lies between 2% and 27%.