Gravitational lensing technique reveals supermassive black hole pairs

Artistic impression of gravitationally lensed starlight (orange) by a supermassive black hole binary. The Einstein ring is shown in blue. Credit: Max Planck Society

doi.org/10.1103/1sfl-87t4
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#Gravitational lensing

When two galaxies collide and merge, the supermassive black holes at the center of each end up getting closer and forming a gravitationally bound pair, known as a supermassive black hole binary system

These systems are very important for understanding galaxy evolution and represent some of the most powerful sources of gravitational waves in the universe.

Although we already have evidence of distant pairs of black holes, scientists have not yet been able to definitively detect those that orbit very close together, rapidly spinning around each other.

A new study, published in the journal “Physical Review Letters,” proposes a clever way to find these hidden systems using a phenomenon called gravitational lensing.

Supermassive black holes are so massive and compact that they act as true natural lenses in space: they strongly bend the light that passes near them.

When a distant star is nearly aligned behind a black hole, its light can be dramatically amplified, making it much brighter for a short period.In the case of a single black hole, this extreme effect only occurs in very precise alignments.

But when two black holes orbit each other, they act like a pair of lenses.

Together, they create a special diamond-shaped structure called a caustic curve, where the light amplification can be even more dramatic.

As these black holes are in orbital motion, the caustic curve rotates and changes shape over time.

As the pair loses energy emitting gravitational waves, the orbit gradually shrinks and accelerates.

This causes the amplification region to sweep across a larger area behind the system.

If a bright star is in this zone, it can produce intense flashes of light that repeat whenever the caustic curve passes over it.

These periodic flashes would be a very characteristic signature of an evolving supermassive black hole binary system.

Furthermore, the interval between the flashes, the peak brightness, and the small variations in these patterns reveal valuable information: the mass of the two black holes, the size of their orbits, and how they are changing over time due to the emission of gravitational waves.

Researchers believe that large-field telescopes and surveys that are coming online, such as the Vera C.

Rubin Observatory and the Nancy Grace Roman Space Telescope, will be able to detect these repeated flashes in the coming years.

Observing these signals would allow the study of the merger of supermassive black holes long before gravitational wave detectors in space, such as the future LISA, become operational.

This possibility opens doors to multi-messenger astronomy, combining light and gravitational waves, and could help test the theory of gravity and the physics of black holes in completely new and exciting ways.

Published in 02/14/2026 22h56

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Text adapted by AI (Grok) and translated via Google API in the English version. Images from public image libraries or credits in the caption. Information about DOI, author and institution can be found in the body of the article.

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