# What we can learn from multi-band observations of black hole binaries

## TL;DR

Multi-band gravitational-wave observations from space and ground can significantly enhance our understanding of black hole binaries by breaking parameter degeneracies and increasing detection rates, especially for high-mass systems.

## Contribution

This paper highlights the benefits of combined space and ground-based GW observations for better parameter estimation and increased detection of black hole binaries.

## Key findings

- Combining data sets improves binary parameter estimation.
- Multi-band observations can increase detected binaries by a factor of 4-7.
- Space-based data can reveal sub-threshold signals in archived ground data.

## Abstract

The LIGO/Virgo gravitational-wave (GW) interferometers have to-date detected ten merging black hole (BH) binaries, some with masses considerably larger than had been anticipated. Stellar-mass BH binaries at the high end of the observed mass range (with "chirp mass" ${\cal M} \gtrsim 25 M_{\odot}$) should be detectable by a space-based GW observatory years before those binaries become visible to ground-based GW detectors. This white paper discusses some of the synergies that result when the same binaries are observed by instruments in space and on the ground. We consider intermediate-mass black hole binaries (with total mass $M \sim 10^2 -10^4 M_{\odot}$) as well as stellar-mass black hole binaries. We illustrate how combining space-based and ground-based data sets can break degeneracies and thereby improve our understanding of the binary's physical parameters. While early work focused on how space-based observatories can forecast precisely when some mergers will be observed on the ground, the reverse is also important: ground-based detections will allow us to "dig deeper" into archived, space-based data to confidently identify black hole inspirals whose signal-to-noise ratios were originally sub-threshold, increasing the number of binaries observed in both bands by a factor of $\sim 4 - 7$.