New method could reveal hidden supermassive black hole binaries

(Press-News.org) Researchers at Oxford University and the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) are proposing a new way to observe tightly bound supermassive black hole binaries. Formed naturally when galaxies merge, only widely separated systems have confidently been observed to date. In a paper published today in Physical Review Letters, the researchers suggest hunting down the hidden systems by searching for repeating flashes of light from individual stars lying behind the black holes as they are temporarily magnified by gravitational lensing as the binary orbits. 

Supermassive black holes reside at the centres of most galaxies. When two galaxies collide and merge, their central black holes eventually form a bound pair, known as a supermassive black hole binary. These systems play a crucial role in galaxy evolution and are among the most powerful sources of gravitational waves in the Universe. While future space-based gravitational-wave observatories will be able to probe such binaries directly, researchers are now showing that they may already be detectable using existing and upcoming electromagnetic surveys.

‘Supermassive black holes act as natural telescopes,’ said Dr Miguel Zumalacárregui from the Max Planck Institute for Gravitational Physics. ‘Because of their enormous mass and compact size, they strongly bend passing light. Starlight from the same host galaxy can be focused into extraordinarily bright images, a phenomenon known as gravitational lensing.’

For a single supermassive black hole, extreme lensing occurs only when a star lies almost exactly along the line of sight. In contrast, a supermassive black hole binary acts as a pair of lenses. This produces a diamond-shaped structure, known as a caustic curve, along which stars can experience dramatic magnification. In theory, the magnification becomes infinite for a point-like source; in practice, it is limited by the finite size of the star.

‘The chances of starlight being hugely amplified increase enormously for a binary compared to a single black hole,’ said Professor Bence Kocsis from the University of Oxford’s Department of Physics and a co-author of the study.

A further key difference is that black hole binaries are not static. The pair orbits under gravity, and according to Einstein’s theory of general relativity, the system slowly loses energy by emitting gravitational waves. As a result, the binary separation shrinks over time and the orbit gradually speeds up.

Graduate student Hanxi Wang is in Professor Kocsis’ group and led the study: ‘As the binary moves, the caustic curve rotates and changes shape, sweeping across a large volume of stars behind it. If a bright star lies within this region, it can produce an extraordinarily bright flash each time the caustic passes over it. This leads to repeating bursts of starlight, which provide a clear and distinctive signature of a supermassive black hole binary.’

The researchers show that the timing and brightness of these bursts are not random. As the binary inspirals, gravitational-wave emission subtly alters the caustic structure, imprinting a characteristic modulation in both the frequency and peak brightness of the flashes. By measuring these patterns, astronomers could infer key properties of the underlying black hole binary, including its masses and orbital evolution.

With powerful wide-field surveys coming online such as the Vera C Rubin Observatory and the Nancy Grace Roman Space Telescope, researchers are optimistic that such repeating lensing bursts could be observed in coming years.

‘The prospect of identifying inspiraling supermassive black hole binaries years before future space-based gravitational wave detectors come online is extremely exciting,’ concludes Professor Kocsis. ‘It opens the door to true multi-messenger studies of black holes, allowing us to test gravity and black hole physics in entirely new ways.’

Notes to editors:

For media enquiries and interview requests, contact Hanxi Wang hanxi.wang@physics.ox.ac.uk

The study ‘Black holes as telescopes: Discovering supermassive binaries through quasi-periodic lensed starlight’ has been published today (12 February) in Physical Review Letters

DOI: 10.1103/1sfl-87t4

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