Radio telescopes over 23 years reveal two particle jets pointing to moving black holes that could merge in 100 years

Supermassive black holes in Mrk 501: a long-sought close pair

Supermassive black holes, with masses ranging from 100 million to 1 billion times that of the Sun, remain among the Universe’s most enigmatic objects. Researchers think they grow largely through mergers, yet a tight pair of supermassive black holes had not previously been directly identified. A new study has now delivered that evidence in the galaxy Markarian 501, thanks to an international team led by Silke Britzen of the Max Planck Institute for Radio Astronomy (MPIfR).

23 years of radio data reveal two near-light-speed particle jets

By combining observations from a network of radio telescopes and examining high-quality, multi-frequency measurements gathered over 23 years, the researchers identified two powerful particle jets travelling at close to the speed of light. One jet is pointed towards Earth and therefore appears brighter, while the second is oriented differently and proved much harder to pick out.

A second jet’s cyclic motion implies an orbiting binary

When the team analysed the long-term behaviour, they found the fainter, second jet traces an anticlockwise path around the larger black hole. The motion repeats in a cycle, which the researchers interpret as the signature of orbital movement within a binary system of two supermassive black holes.

Einstein ring in June 2022 confirms alignment and gravitational lensing

In June 2022 the system’s emission took on the appearance of an Einstein ring: light from the second jet was bent by the first black hole, producing an almost perfect circle. This gravitational lensing effect strengthened the case for two black holes, because the geometry requires their system to be exceptionally well aligned with the observer.

Orbit of about 121 days and a possible merger in 100 years

The study estimates that the black holes revolve around one another with a period of roughly 121 days, separated by 250–540 astronomical units-remarkably small for objects of this scale. If the present dynamics persist, the pair could merge in approximately 100 years. Such an event would generate low-frequency gravitational waves that could, in principle, be detected by radio-based efforts such as the Pulsar Timing Array (PTA). In that scenario, Mrk 501 could become crucial for linking observed gravitational-wave background signals to a specific supermassive binary system.

Why jets matter when the Event Horizon Telescope cannot resolve the pair

Detecting the pair indirectly through their particle jets is particularly significant because even the Event Horizon Telescope-which in 2019 and 2022 produced landmark images of black holes-does not have sufficient resolution to image two separate objects in Mrk 501. The finding opens a rare opportunity to probe the final stage of supermassive black-hole coalescence and to test theoretical models of how such systems form and evolve.

“Если гравитационные волны будут зарегистрированы, то мы сможем наблюдать, как их частота постепенно растёт по мере спирального сближения двух гигантов, предоставляя редкую возможность увидеть слияние сверхмассивных чёрных дыр в реальном времени”, - отметил соавтор исследования Эктор Оливари (Hector Olivares).