Astronomers may have found a new way to spot one of the universe’s most elusive objects: pairs of supermassive black holes locked in a tight orbit around each other. Rather than waiting for future gravitational-wave observatories, researchers say these hidden systems could reveal themselves through repeating flashes of starlight.
The idea comes from a study led by scientists at the University of Oxford and the Max Planck Institute for Gravitational Physics. Published in Physical Review Letters, the research suggests that the powerful gravity of two orbiting supermassive black hole bcan repeatedly brighten the light of stars behind them, creating a distinctive signal that astronomers may be able to detect.
Finding these close black hole pairs has long been a challenge. While astronomers have already discovered some supermassive black holes orbiting far apart, the tighter systems expected to form after galaxy mergers are much harder to identify. Yet these binaries are considered key pieces in the story of how galaxies grow and evolve.
A Natural Outcome Of Galaxy Mergers
Nearly every large galaxy is believed to host a supermassive black hole at its center. When two galaxies collide and eventually merge, their central black holes can become trapped by each other’s gravity, forming what astronomers call a supermassive black hole binary. Scientists expect these systems to be relatively common across the universe, but spotting them is another matter. The closer the two black holes get, the more difficult they become to observe directly.
As explained in the study, available on Physical Review Letters, these binaries should also be major sources of gravitational waves. Future space missions are expected to detect those signals directly. Until then, researchers are looking for other ways to find the hidden pairs.
One possibility comes from gravitational lensing, the process by which massive objects bend and focus light.
“Supermassive black holes act as natural telescopes,” said Dr. Miguel Zumalacárreguiof 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.”
A Cosmic Light Show Powered By Gravity
A single black hole can magnify the light from a background star, but only when the alignment between the star, the black hole and the observer is extremely precise. A binary system changes the situation. According to the study, two orbiting cosmic giants create a much larger region where strong magnification can occur. The combined gravitational field generates a diamond-shaped structure known as a caustic curve.
When a star passes through this region, its light can brighten dramatically. While theoretical calculations show that a perfectly point-like star could be magnified infinitely, real stars have finite sizes that place a limit on the effect.
“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.
This means astronomers have a better chance of catching these events than they would if they were searching for lensing caused by a lone supermassive black hole.
Repeating Flashes May Reveal Hidden Black Holes
What makes the proposed signal especially interesting is that it should repeat. As the two black holes orbit each other, they slowly lose energy through gravitational-wave emission. Their orbit gradually shrinks, and the caustic curve rotates and changes shape as the system evolves.
The study’s lead author, Hanxi Wang, explained that this moving lensing structure sweeps across stars located behind the binary. Each passage can trigger a burst of light.
“As the binary moves, the caustic curve rotates and changes shape, sweeping across a large volume of stars behind it,” Wang said. “If a bright star lies within this region, it can produce an extraordinarily bright flash each time the caustic passes over it.”
The result is a series of recurring flashes rather than a one-time event. Based on the latest research, the timing and brightness of those flashes should follow recognizable patterns. By tracking them, astronomers could learn more about the masses of the black holes and how their orbit is changing.
New observatories such as the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope are expected to help search for these repeating signals. If successful, the method could provide a new way to uncover supermassive black hole binaries that have remained hidden despite years of observations.
