Bright flashes of lensed starlight guide the way
Artistic impression of gravitationally lensed starlight (orange) by a supermassive black hole binary. The Einstein ring is shown in blue.
© 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.
Supermassive black hole binaries form naturally when galaxies merge, but scientists have only confidently observed a very few of these systems that are widely separated. Black hole binaries that closely orbit each closer have not yet been measured. 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.
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."
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. As they orbit each other under the influence of gravity, they slowly lose energy through the emission of gravitational waves. This also reduces the distance between the two black holes. By measuring these flashes of light, astronomers could deduce important properties of the binary system, including not only the masses of the black holes, but also their evolution in their orbits. While the flashes of light could repeat on timescales of a few years, which corresponds to the orbital period of such particularly heavy black holes, it takes significantly longer for the frequency to change. What is possible are snapshots: different pairs of black holes observed in this way would show different frequencies, as these centers of different galaxies are also at different stages of development.
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."
