Prospects of gravitational-waves detections from common-envelope evolution with LISA

TL;DR

This paper assesses the potential for LISA to detect gravitational waves from the common-envelope phase in binary star evolution, highlighting the detectability prospects, expected source counts, and methods to distinguish these signals from other sources.

Contribution

It introduces a framework for detecting and identifying gravitational waves from common-envelope evolution using LISA, focusing on the self-regulated phase and parameter space mapping.

Findings

Estimated 0.1-100 sources in the Galaxy during LISA's mission.

Signals with SNR > 10 are detectable if the CE stalls at certain separations.

Distinctive GW braking-index can help differentiate CE signals from other sources.

Abstract

Understanding common envelope (CE) evolution is an outstanding problem in binary evolution. Although the CE phase is not driven by gravitational-wave (GW) emission, the in-spiraling binary emits GWs that passively trace the CE dynamics. Detecting this GW signal would provide direct insight into the gas-driven physics. Even a non-detection might offer invaluable constraints. We investigate the prospects of detection of a Galactic CE by LISA. While the dynamical phase of the CE is likely sufficiently loud for detection, it is short and thus rare. We focus instead on the self-regulated phase that proceeds on a thermal timescale. Based on population synthesis calculations and the (unknown) signal duration in the LISA band, we expect $\sim 0.1-100$ sources in the Galaxy during the mission duration. We map the GW observable parameter space of frequency $f_\mathrm{GW}$ and its derivative $\dot f_\mathrm{GW}$ remaining agnostic on the specifics of the inspiral, and find that signals with $\mathrm{SNR}>10$ are possible if the CE stalls at separations such that $f_\mathrm{GW}\gtrsim2\times10^{-3}\,\mathrm{Hz}$. We investigate the possibility of misidentifying the signal with other known sources. If the second derivative $\ddot f_\mathrm{GW}$ can also be measured, the signal can be distinguished from other sources using a GW braking-index. Alternatively, coupling LISA with electromagnetic observations of peculiar red giant stars and/or infrared and optical transients might allow for the disentangling of a Galactic CE from other Galactic and extra-galactic GW sources.