First close pair of supermassive black holes detected in Markarian 501

For the first time in the history of astronomy, researchers have found direct evidence of two supermassive black holes at the center of a single galaxy, orbiting each other at an extraordinarily close distance — and possibly on the verge of merging within the next hundred years.

The discovery was made by a team led by Silke Britzen of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, who analyzed the galaxy Markarian 501 (Mrk 501) in the constellation Hercules. Their findings were published in the Monthly Notices of the Royal Astronomical Society.

A second jet hiding in plain sight

It has long been known that the supermassive black hole at the core of Mrk 501 produces a powerful jet of particles moving at nearly the speed of light, pointing almost directly toward Earth — which is why it appears so bright to observers. What nobody expected was a second jet.

Using high-resolution radio observations collected over approximately 23 years and across multiple frequencies, the research team detected not one but two jets emanating from the galactic nucleus. This is the first direct image of such a configuration at the center of a galaxy, and it constitutes direct evidence of the presence of a second supermassive black hole orbiting the first.

«We searched for it for so long, and then it came as a complete surprise that we could not only see a second jet, but even track its movement,» said Britzen.

The second jet is oriented differently from the first, which made it far more difficult to detect. Over just a few weeks of observation, astronomers confirmed that the second jet starts behind the larger black hole and moves counterclockwise around it in a repeating pattern — a direct consequence of the orbital motion of the binary system.

A dance measured in weeks and centuries

By analyzing the long-term brightness evolution of the jets, the team determined that the two black holes orbit each other with a period of approximately 121 days. Their separation is estimated at 250 to 540 times the Earth-Sun distance — an extraordinarily small gap for objects with masses ranging between 100 million and one billion solar masses.

At that proximity, the orbital separation is shrinking. Depending on the actual masses involved, the two black holes could coalesce in as little as 100 years — an eyeblink on cosmic timescales.

In June 2022, an especially striking observation confirmed the binary nature of the system: the radiation reached Earth along such a curved path that it formed a ring-shaped structure, a so-called Einstein ring. The most likely explanation is that the two black holes were momentarily aligned with our line of sight, causing the gravity of the foreground black hole to lens the light of the second jet behind it.

Gravitational waves on the horizon

The vast distance between Mrk 501 and Earth means that no existing telescope — not even the Event Horizon Telescope (EHT), which captured the first images of black hole shadows in 2019 and 2022 — can resolve the two objects separately. Nevertheless, the system is expected to emit gravitational waves at very low frequencies as the orbit tightens, placing it within the detection range of pulsar timing arrays (PTAs).

Supermassive black hole binaries (SMBHBs) are already considered the leading explanation for the gravitational wave background signal detected in 2023 by the European Pulsar Timing Array. Mrk 501 is now a prime candidate for attributing a specific gravitational wave source to a known binary system.

«If gravitational waves are detected, we may even see their frequency steadily rise as the two giants spiral toward collision, offering a rare chance to watch a supermassive black hole merger unfold,» noted co-author Héctor Olivares.

A window into galaxy evolution

Supermassive black holes are believed to reside at the centers of nearly all large galaxies, with masses ranging from millions to billions of times that of the Sun. Accumulating such enormous mass through gas accretion alone would take longer than the age of the universe — meaning mergers between galaxies, and between their central black holes, must play a fundamental role in their growth.

Despite galaxy collisions being commonplace on cosmic timescales, no close pair of supermassive black holes had ever been directly imaged before this discovery. Mrk 501 now provides a unique observational laboratory to study one of the most consequential processes in the evolution of galaxies.

The full study is available in the Monthly Notices of the Royal Astronomical Society: S. Britzen et al., «Detection of a second jet within the nuclear core of Mrk 501,» MNRAS (2026). DOI: 10.1093/mnras/stag291.

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