Double Compact Objects as Low-frequency Gravitational Wave Sources

TL;DR

This study assesses the potential for detecting double compact object binaries in the Milky Way with LISA, finding that only a small fraction, mainly NS-NS systems, are detectable due to their orbital properties and evolutionary uncertainties.

Contribution

It provides the first detailed estimates of low-frequency gravitational wave sources from Galactic double compact objects considering different evolutionary models.

Findings

Only a few binaries are detectable with LISA, mostly NS-NS systems.

Detection depends heavily on the evolutionary model, especially the treatment of the common envelope phase.

Most binaries are undetectable due to high eccentricities and long orbital periods.

Abstract

We study the Galactic field population of double compact objects (NS-NS, BH-NS, BH-BH binaries) to investigate the number (if any) of these systems that can potentially be detected with LISA at low gravitational-wave frequencies. We calculate the Galactic numbers and physical properties of these binaries and show their relative contribution from the disk, bulge and halo. Although the Galaxy hosts 10^5 double compact object binaries emitting low-frequency gravitational waves, only a handful of these objects in the disk will be detectable with LISA, but none from the halo or bulge. This is because the bulk of these binaries are NS-NS systems with high eccentricities and long orbital periods (weeks/months) causing inefficient signal accumulation (small number of signal bursts at periastron passage in 1 yr of LISA observations) rendering them undetectable in the majority of these cases. We adopt two evolutionary models that differ in their treatment of the common envelope phase that is a major (and still mostly unknown) process in the formation of close double compact objects. Depending on the adopted evolutionary model, our calculations indicate the likely detection of about 4 NS-NS binaries and 2 BH-BH systems (model A; likely survival of progenitors through CE) or only a couple of NS-NS binaries (model B; suppression of the double compact object formation due to CE mergers).