Novel Method Detects Pairs of the Biggest Black Holes

In a study published in Nature Astronomy, an international team of astrophysicists, including the University of Zurich, proposes a new method for detecting pairs of the largest black holes discovered at the centers of galaxies by analyzing gravitational waves produced by binaries of nearby small stellar black holes.

One of the greatest mysteries in astronomy is still the formation of supermassive black holes located in the centers of galaxies. They could have originated when the universe was still extremely young and have always been huge.

They grew most likely by accreting matter and other black holes throughout time. In the process of consuming another enormous black hole, a supermassive black hole releases gravitational waves, which are ripples in spacetime that travel across the universe.

The Challenge of Detecting Massive Black Holes

Recent detections of gravitational waves have only come from tiny black holes, the remains of stars. It is now difficult to detect the signals of individual pairs of massive black holes because current detectors do not detect the extremely low gravitational wave frequencies that these objects generate.

This will be partly addressed by planned future detectors, including the space-based LISA project led by ESA, but it will remain unable to detect the most enormous black hole couples.

Use High Frequencies to Measure Lower Frequencies

An international team of astrophysicists led by former University of Zurich students proposes a new idea and method for detecting pairs of the largest black holes found at the center of galaxies by analyzing gravitational waves generated by binaries of nearby small stellar black holes, which are the remnants of collapsed stars.

This strategy, necessitating using a deci-Hz gravitational wave detector, might allow the discovery of the largest supermassive black hole binaries that would otherwise be unreachable.

Our idea basically works like listening to a radio channel. We propose to use the signal from pairs of small black holes similar to how radio waves carry the signal. The supermassive black holes are the music that is encoded in the frequency modulation (FM) of the detected signal. The novel aspect of this idea is to utilize high frequencies that are easy to detect to probe lower frequencies that we are not sensitive to yet.

Jakob Stegmann, Study Lead Author and Postdoctoral Research Fellow, Max Planck Institute for Astrophysics

A Beacon Indicates Larger Black Holes

Recent findings from pulsar timing arrays indicate the presence of merging supermassive black hole binaries. This evidence, however, is indirect and derived from the combined signal of several distant binaries, which effectively generate background noise.

The suggested approach for detecting individual supermassive black hole binaries takes advantage of the slight variations they create in gravitational waves released by two nearby small stellar-mass black holes. The small black hole binary, therefore, serves as a beacon, showing the presence of larger black holes.

Scientists might discover previously unknown supermassive black hole binaries with masses ranging from 10 million to 100 million times that of the Sun by detecting minuscule modulations in signals from small black hole binaries, even at large distances.

As the path for the Laser Interferometer Space Antenna (LISA) is now set, after adoption by ESA last January, the community needs to evaluate the best strategy for the following generation of gravitational wave detectors, in particular, which frequency range they should target – studies like this bring a strong motivation to prioritize a deci-Hz detector design.

Lucio Mayer, Study Co-Author and Black Hole Theorist, University of Zurich

Journal Reference:

Stegmann, J., et. al. (2024) Imprints of massive black-hole binaries on neighboring decihertz gravitational-wave sources. Nature Astronomy. doi:10.1038/s41550-024-02338-0