# Detectability of intermediate-mass black holes in multiband gravitational wave astronomy

## Abstract

The direct measurement of gravitational waves is a powerful tool for surveying the population of black holes across the universe. While LIGO has detected black holes as heavy as ~50 M (ref. 1), there is not yet unambiguous evidence of black holes in the intermediate mass range of 102–5M. Recent electromagnetic observations have hinted towards the existence of intermediate-mass black holes (IMBHs)2,3,4; however, their masses remain poorly constrained. Here we argue that multiband observations by space- and ground-based gravitational wave detectors5,6 will be able to survey a broad population of IMBHs at cosmological distances. By utilizing general relativistic simulations of merging black holes7 and state-of-the-art gravitational waveform models8, we classify three distinct populations of binaries with IMBHs and discuss what can be observed about each. Multiband observations involving the upgraded LIGO detector9 and the proposed space-mission LISA10 would detect the inspiral, merger and ring-down of IMBH binaries out to redshift ≈ 2, pushing out to redshift ≈ 5 if next-generation ground-based detectors11,12 are operational at the same time. To facilitate studies of multiband IMBH sources, we provide analytic relations for the maximum redshift of multiband detectability, as a function of black hole mass, for various detector combinations. Our study paves the way for future work on what can be learned from IMBH observations in the era of multiband gravitational wave astronomy.

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## Data availability

The data products that support the findings of this study are available via Zenodo at https://doi.org/10.5281/zenodo.3364249.

## Code availability

The scripts that support the findings of this study are available via Zenodo at https://doi.org/10.5281/zenodo.3364249.

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## Acknowledgements

We thank L. Cadonati and C. Berry for useful discussions, and L. London and S. Khan for the links to generate their waveform model in the LIGO Analysis Library. K.J. and D.S. were funded by NASA grant no. 80NSSC19K0322 and NSF grant nos PHY 1806580 and 1809572. The work by C.C. was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. C.C. also acknowledges support from NSF grant no. PHY-1708212.

## Author information

All authors contributed equally to the text and the primary results.

Correspondence to Karan Jani.

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### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1 and 2 and refs. 1–7.

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Jani, K., Shoemaker, D. & Cutler, C. Detectability of intermediate-mass black holes in multiband gravitational wave astronomy. Nat Astron (2019). https://doi.org/10.1038/s41550-019-0932-7