What black hole mergers sound like and why It matters

The frequency of the gravitational waves increases as the black holes spiral closer together, creating a characteristic chirp signal that can be measured on Earth. By analyzing this signal, scientists can determine the chirp mass, a combination of the two individual black hole masses.

Until now, it was assumed that the chirp mass could vary depending on the properties of the black holes. However, the HITS team’s models suggest that there are some standard black hole masses that result in universal chirp masses. “The existence of universal chirp masses not only tells us how black holes form”, says Fabian Schneider, who led the study at HITS, “it can also be used to infer which stars explode in supernovae.” This could help solve some of the mysteries surrounding the supernova mechanism, the nuclear and stellar physics involved, and the accelerated expansion of the Universe.

What envelope stripping reveals about black hole formation

The key to understanding the universal chirp masses lies in the envelope-stripping process that occurs in binary star systems, where two stars orbit each other and exchange mass. This process affects the final fate of the stars, making them more likely to explode in supernovae or collapse into black holes. The HITS team found that envelope-stripped stars form black holes of less than 9 or more than 16 solar masses, but almost none in between.

This implies that merging black holes with these standard masses will produce universal chirp masses of roughly 8 and 14 solar masses. “Any features in the distributions of black-hole and chirp masses can tell us a great deal about how these objects have formed”, says Philipp Podsiadlowski from Oxford University, second author of the study and currently Klaus Tschira guest professor at HITS.

Gravitational waves could solve supernova mysteries

The researchers also noted that there seems to be a gap in the observed chirp masses of merging binary black holes, as well as an overabundance at exactly the universal values predicted by their models. “Because the number of observed black-hole mergers is still rather low, it is not clear yet whether this signal in the data is just a statistical fluke or not”, says Fabian Schneider. The team hopes that future gravitational-wave observations will confirm their predictions and shed more light on the universal sounds of black hole mergers.

The study was published in The Astrophysical Journal Letters

Study Abstract

In binary black hole mergers from isolated binary-star evolution, both black holes are from progenitor stars that have lost their hydrogen-rich envelopes by binary mass transfer. Envelope stripping is known to affect the pre-supernova core structures of such binary-stripped stars and thereby their final fates and compact remnant masses. In this paper, we show that binary-stripped stars give rise to a bimodal black hole mass spectrum with characteristic black hole masses of about 9 M⊙ and 16 M⊙ across a large range of metallicities. The bimodality is linked to carbon and neon burning becoming neutrino dominated, which results in interior structures that are difficult to explode and likely lead to black hole formation. The characteristic black hole masses from binary-stripped stars have corresponding features in the chirp-mass distribution of binary black hole mergers: peaks at about 8 and 14 M⊙ and a dearth in between these masses. Current gravitational-wave observations of binary black hole mergers show evidence for a gap at 10–12 M⊙ and peaks at 8 and 14 M⊙ in the chirp-mass distribution. These features are in agreement with our models of binary-stripped stars. In the future, they may be used to constrain the physics of late stellar evolution and supernova explosions and may even help measure the cosmological expansion of the universe.