Astronomers from the University of Oxford and the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) have outlined a new strategy to discover one of the most elusive objects in the universe: a pair of tightly coupled supermassive black holes.
These giant black hole pairs are expected to form naturally after galaxies merge. Astronomers have identified several pairs of widely separated supermassive black holes, but finding pairs of black holes orbiting closer together has proven much more difficult.
In a study published in physical review letterthe researchers suggest looking for characteristic signals. As black holes orbit each other, their massive gravity can repeatedly magnify the light from the stars behind them, creating repeated flashes that can reveal hidden systems.
Galaxy merger creates supermassive black hole binary
A supermassive black hole exists at the center of most galaxies. When galaxies collide and eventually merge, their central black holes can become gravitationally bound, forming what scientists call supermassive black hole binaries.
These systems are important for understanding how galaxies evolve over time. It is also expected to generate some of the strongest gravitational waves in the universe.
Future space-based gravitational-wave observatories should be able to detect these binaries directly. But new research suggests astronomers may not have to wait. Existing and future sky surveys have the potential to identify them through their impact on visible light.
“Supermassive black holes act as natural telescopes,” said Dr. Miguel Zumaracaregi of the Max Planck Institute for Gravitational Physics. “Because of their huge mass and compact size, they strongly bend the light passing through them. Starlight from the same host galaxy can be focused into very bright images, a phenomenon known as gravitational lensing.”
How gravitational lenses produce bright flashes
A single supermassive black hole can dramatically magnify background stars, but only if the alignment is nearly perfect.
Binary systems work differently. Because the two black holes act as gravitational lenses, the area where extreme magnification occurs is even larger. This pair creates a diamond-shaped feature known as a caustic curve, which can cause the star to appear dramatically brighter.
In theory, a perfectly point-like star can expand infinitely. In reality, there is a limit to the size of a star, so there is a limit to how bright the effect can be.
“The potential for significant amplification of starlight is greatly increased in binary stars compared to single black holes,” said Professor Bence Kocsis of the University of Oxford’s Department of Physics and co-author of the study.
Repeated flashes of stars could reveal hidden black holes
Unlike a single black hole, black hole binaries are constantly changing.
As two black holes orbit each other, they gradually lose energy through the emission of gravitational waves, a process predicted by Einstein’s theory of general relativity. Over time, this causes the black holes to move closer together and orbit faster.
Graduate student Hanxi Wang, a member of Professor Kocsis’ group, led the research. “As the binary moves, the caustic rotates and changes shape, sweeping away a large amount of stars behind it. If a bright star is within this region, it can produce a very bright flash each time the caustic passes over it. This results in repeated bursts of starlight, giving the supermassive black hole binary its distinct and distinctive signature.”
Because the caustic structure changes continuously, the resulting flashes occur many times, creating recognizable patterns that astronomers can explore.
Clues about black hole mass and orbit
The researchers found that the timing and intensity of these flashes should follow a predictable trend, rather than appearing randomly.
Gravitational waves cause the orbit to slowly contract, causing subtle changes in the shape and motion of the caustic curve. These changes leave measurable traces in both the brightness and frequency of the flash.
By analyzing these patterns, researchers may be able to deduce important features of the hidden binary, such as the black hole’s mass and details of its orbital evolution.
Powerful new observatories such as the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope are expected to dramatically expand the search for these recurring lensing phenomena in the coming years.
“The prospect of identifying an inspirational supermassive black hole binary years before future space-based gravitational wave detectors become operational is extremely exciting,” Professor Kocsis concluded. “This opens the door to true multi-messenger studies of black holes, allowing us to test gravity and black hole physics in entirely new ways.”
