The known jet points toward Earth, so it looks bright and easy to study. The new jet points elsewhere. That's why it hid for so long.

Takeaway: a decades-long radio archive exposed a jet nobody expected, and with it a likely second giant black hole.

What does the motion reveal?

Over just a few weeks, the second jet shifted dramatically. It appeared to emerge from behind the larger black hole and sweep counterclockwise around it. The pattern repeated, pointing to regular orbital motion of the whole system.

"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.

The team also flagged an unusual event in June 2022, when radiation from the system formed a ring. That's an Einstein ring — gravity bending light into a ringlike shape. One explanation: the two black holes and Earth briefly lined up, letting the foreground black hole act as a lens for light from the jet behind it. The effect, called gravitational lensing, adds another clue.

Takeaway: the swaying jets and a fleeting ring both fit a two-black-hole system in orbital lockstep.

How soon could they merge?

From changes in the jets and repeating brightness patterns, the team calculated the 121-day orbital period. The estimated separation is just 250 to 540 times the Earth-Sun distance. Enormous by everyday standards, but remarkably cramped for objects weighing 100 million to a billion Suns.

That tight orbit matters. Depending on the actual masses, the separation may shrink fast enough for a merger in as little as 100 years. By astronomical standards, that's an exceptionally short countdown, and it could give researchers a rare window onto how supermassive black holes approach their final collision.

Takeaway: two cosmic giants, orbiting in months rather than millennia, may be on a 100-year collision clock.

Can anyone actually watch it happen?

Not directly, at least not yet. Mrk 501 is so distant that even the Event Horizon Telescope — which produced the first black hole images in 2019 and 2022 — cannot resolve this pair as two objects.

But there's another route. Orbiting giants generate gravitational waves, tiny ripples in spacetime that travel across the universe. This pair should produce extremely low-frequency waves detectable with pulsar timing arrays, or PTAs. These arrays use the metronome-like radio pulses of rapidly spinning neutron stars, called pulsars, as cosmic clocks. Subtle shifts in when those pulses arrive at Earth betray passing waves.

Supermassive black hole binaries are already the leading explanation for the gravitational wave background, a persistent hum from many distant sources. In 2023, the European Pulsar Timing Array and other collaborations reported evidence of that background. Mrk 501 now becomes a prime target for tying a gravitational wave signal to one specific pair — and for measuring whether the orbit is truly shrinking toward merger.

"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," said co-author Héctor Olivares.

Takeaway: pulsar clocks, not photographs, could catch the final approach — if the signal shows up.

What are the caveats?

The findings await formal publication, and the black holes themselves remain unresolved as individual objects. The 100-year figure depends on the pair's actual masses, which carry wide uncertainty. The lensing interpretation of the June 2022 event is one possible explanation, not a confirmed one.

Still, a candidate binary this tight, this close to merger, doesn't come along often. The next signals may arrive not in images, but in the ticking of dead stars.