Artistic impression of gravitationally lensed starlight (orange) by a supermassive black hole binary. The Einstein ring is shown in blue.Credit: Physics simulation enhanced using AI
To the point:
- New method: Researchers at Oxford University and the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in the Potsdam Science Park propose a new way to detect supermassive black hole binaries using gravitational lensing.
- Gravitational lensing: Black holes act as natural telescopes, bending light with their gravity. This magnification creates bright images of stars from the same galaxy that lie behind the supermassive black hole binary.
- Detectable signals: As the binary orbits, it produces repeating flashes of lensed starlight. Current and upcoming wide-field surveys may detect these bursts in the future. These bursts can provide information about the black holes’ properties and enabling entirely new studies.
Bright flashes of lensed starlight guide the way
New method
Tightly bound supermassive black hole binaries form naturally when galaxies merge, but 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 centers 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 like LISA 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 reside at the centers 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 like LISA will be able to probe such binaries directly, researchers are now showing that they may already be detectable using existing and upcoming electromagnetic surveys.
Gravitational lensing
“Supermassive black holes act as natural telescopes,” says 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, extremely strong 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.
“The chances of starlight being hugely amplified increase enormously for a binary compared to a single black hole,” explains 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. While the pair orbits under gravity the system slowly loses energy by emitting gravitational waves. As a result, the binary separation shrinks over time and the orbit gradually speeds up.
“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,” says Hanxi Wang, a PhD student in Kocsis’ group who led the study “This leads to repeating bursts of starlight, which provide a clear and distinctive signature of a supermassive black hole binary.”
For a single supermassive black hole, extremely strong 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.
“The chances of starlight being hugely amplified increase enormously for a binary compared to a single black hole,” explains 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. While the pair orbits under gravity the system slowly loses energy by emitting gravitational waves. As a result, the binary separation shrinks over time and the orbit gradually speeds up.
“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,” says Hanxi Wang, a PhD student in Kocsis’ group who led the study “This leads to repeating bursts of starlight, which provide a clear and distinctive signature of a supermassive black hole binary.”
Valuable information from detectable signals
The researchers show that the timing and brightness of these bursts encode valuable information about the black hole binary. 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 the 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 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.”
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 the 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 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.”
Media contact:
Dr. Elke Müller
Press Officer AEI Potsdam, Scientific Coordinator
Tel: +49 331 567-7303
Email: elke.mueller@aei.mpg.de
Science contact:
Dr. Miguel Zumalacarregui
Group; Leader
Tel: +49 331 567-7322
Fax: +49 331 567-7298
Email: miguel.zumalacarregui@aei.mpg.de
Publication
Wang, H.; Zumalacarregui, M.; Kocsis, B.
Black holes as telescopes: Discovering supermassive binaries through quasi-periodic lensed starlight. Physical Review Letters 136, 061403 (2026)
MPG.PuRe - pre-print - publisher-version
