Astronomers imaging the core of galaxy Markarian 501 have detected two large jets, providing the first direct evidence of a close pair of supermassive black holes orbiting each other.

A long-term observational campaign spanning more than two decades leads to the first confirmation of a SMBH binary at Markarian 501’s center, with findings accepted for publication in Monthly Notices of the Royal Astronomical Society.

Led by Silke Britzen, the team confirms two distinct black holes through a second relativistic jet, reinforcing the binary interpretation.

The potential merger gives a rare opportunity to observe a supermassive black hole collision in real time via gravitational waves and orbital dynamics as measurements continue to improve.

The jet motions show the second jet lagging behind the primary black hole and moving in a counterclockwise orbit, with occasional Einstein-ring effects from gravitational lensing.

Individual resolution of the two black holes isn’t yet possible due to distance, but the system remains a key source for linking gravitational wave signals to a specific SMBH binary.

The system provides observational support for the tight binary stage in galactic black hole mergers, aligning with theories of galaxy growth through mergers and eventual coalescence.

An Einstein ring observed in June 2022, caused by gravitational lensing, complements the jet data and underpins the binary interpretation.

The ring structure independently corroborates the system’s geometry under lensing, tying the radio jet observations to a bound binary.

Twenty-three years of radio observations enabled the detection of a second jet and ruled out a single-jet explanation, marking a methodological breakthrough in identifying close SMBH binaries.

The pair is nearing merger, potentially within about a century, which would generate low-frequency gravitational waves detectable by pulsar timing arrays.

The two SMBHs are estimated to weigh between 100 million and a billion solar masses, orbiting at roughly 250 to 540 astronomical units with an orbital period around 121 days.