Origin and Implications of high eccentricities in massive black hole binaries at sub-pc scales

TL;DR

This paper investigates the evolution and detection prospects of high eccentricities in massive black hole binaries at sub-parsec scales, highlighting their impact on gravitational wave signals and electromagnetic counterparts.

Contribution

It provides a comprehensive analysis of eccentricity evolution in MBHBs in different environments and discusses implications for GW detection and electromagnetic signals.

Findings

MBHBs can maintain detectable eccentricities near coalescence.

Gas-driven dynamics tend to produce larger eccentricities than stellar environments.

Eccentricities at PTA scales can be quite large, affecting signal modeling.

Abstract

We outline the eccentricity evolution of sub-parsec massive black hole binaries (MBHBs) forming in galaxy mergers. In both stellar and gaseous environments, MBHBs are expected to grow large orbital eccentricities before they enter the gravitational wave (GW) observational domain. We re--visit the predicted eccentricities detectable by space based laser interferometers (as the proposed ELISA/NGO) for both environments. Close to coalescence, many MBHBs will still maintain detectable eccentricities, spanning a broad range from <10^{-5} up to <~ 0.5. Stellar and gas driven dynamics lead to distinct distributions, with the latter favoring larger eccentricities. At larger binary separations, when emitted GWs will be observed by pulsar timing arrays (PTAs), the expected eccentricities are usually quite large, in the range 0.01-0.7, which poses an important issue for signal modelling and detection algorithms. In this window, large eccentricities also have implications on proposed electromagnetic counterparts to the GW signal, which we briefly review.