An international team led by Silke Britzen of the Max Planck Institute for Radio Astronomy has uncovered what the researchers describe as the first direct imaging evidence of two supermassive black holes orbiting extremely close together at the center of a galaxy.

The galaxy, Mrk 501, was long known to host a single black hole launching a particle jet at nearly the speed of light aimed toward Earth.

By analyzing radio observations collected at multiple frequencies over roughly 23 years, the team discovered a second, previously unseen jet pointing in a different direction — a jet they attribute to a second, unobserved black hole.

"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," Britzen said, according to Science Daily.

Supermassive black holes are thought to sit at the centers of nearly all large galaxies, and galaxy collisions should bring their central black holes together into orbiting pairs that eventually merge.

Yet despite abundant evidence of colliding galaxies, scientists have struggled to model the final merger stages and had not reliably identified a closely orbiting pair — making Mrk 501 a rare and valuable catch.

The difficulty is compounded by scale: these objects can weigh millions to billions of times the mass of the Sun, and accreting gas alone may be too slow to explain how the largest ones grow, which is why mergers are considered a likely missing ingredient.

SSBCrack News reports that the binary hypothesis also helps explain the growth problem, since merging with other massive black holes could account for their immense sizes.

Even the Event Horizon Telescope, which imaged black holes in 2019 and 2022, lacks the resolution to resolve this pair individually, according to Science Daily.

The newly identified jet appeared to emerge from behind the larger black hole and sweep counterclockwise around it, a repeating motion suggesting regular orbital movement.

"Evaluating the data felt like being on a ship. The entire jet system is in motion. A system of two black holes can explain this: The orbital plane sways," Britzen explained, per Science Daily.

The team calculated that the pair completes an orbit roughly every 121 days, separated by just 250 to 540 times the Earth–Sun distance — remarkably tight for objects of 100 million to a billion solar masses.

Science Daily also reports an unusual June 2022 event in which radiation from the system formed an Einstein ring, possibly produced when gravity from the foreground black hole lensed light from the jet behind it.

SSBCrack News likewise describes the ring-like effect as a potential clue to the alignment of the two black holes and Earth.

Depending on the black holes' actual masses, their separation may shrink fast enough for a merger in as little as 100 years — an exceptionally short countdown by astronomical standards.

Because Mrk 501 is too distant for any current instrument to watch the gap close directly, the decisive test lies elsewhere: the orbiting giants should emit extremely low-frequency gravitational waves detectable through pulsar timing arrays, which use the steady pulses of rotating neutron stars as cosmic clocks.

Science Daily notes that supermassive black hole binaries are already the leading explanation for the gravitational wave background reported in 2023 by the European Pulsar Timing Array and other collaborations, and Mrk 501 is now a promising target for tying that signal to an individual pair.

"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," co-author Héctor Olivares said.

SSBCrack News reports such a detection could also let scientists track the pair's orbital evolution and anticipate the merger.