Team led by Silke Britzen proposes black hole pair orbiting Markarian 501

A team led by Silke Britzen of the Max Planck Institute for Radio Astronomy in Bonn proposed that two supermassive black holes orbit the center of Markarian 501, according to research reported Nov. 5, 2026. Monthly Notices of the Royal Astronomical Society has accepted the paper for publication. It will appear in an upcoming issue.

The paper lists ten authors, including Britzen, Héctor Olivares and Gopal-Krishna. Britzen leads the international team from the Max Planck Institute for Radio Astronomy in Bonn. The work comes from an international collaboration based at the institute.

The team studied detailed radio observations gathered at frequencies across dozens of separate days spanning roughly 23 years. They gathered data on individual nights spread across more than two decades. The observing campaign returned a long-term record of the source. Researchers used that record to track changes in the jets and to find repeating brightness patterns.

In that record they identified a second jet whose movement they could track. The researchers describe that detection as the first direct imaging evidence of such a system at a galaxy’s center. Britzen said searching for it took so long that seeing the second jet, and following its motion, came as a complete surprise. Britzen also said that in the ideal case, the team would have observed the VLBA data more frequently.

The VLBA is a network of radio telescopes used to gather the observations.

She described evaluating the data as feeling like being on a ship, with the entire jet system in motion and the orbital plane swaying. A system of two black holes can explain this because the orbital plane sways. Over just a few weeks, the second jet appeared to emerge from behind the larger black hole and sweep counterclockwise around it, suggesting regular orbital motion of the whole system.

By examining changes in the jets and repeating brightness patterns, the team calculated that the black holes complete an orbit approximately every 121 days. Their estimated separation is 250 to 540 Earth–Sun distances, remarkably small for objects of 100 million to a billion solar masses.

Depending on their actual masses, the black holes’ separation may shrink rapidly enough for them to merge in as little as 100 years. The galaxy lies so far away that even today’s best tools cannot separate them individually. The Event Horizon Telescope lacks the resolution to capture the pair individually.

Gravitational wave detections from Markarian 501 would let researchers track changes in the pair’s orbit and judge whether the black holes are nearing their final merger. Héctor Olivares of the University of Aveiro, a co-author, said, “If gravitational waves are detected, we may even see their frequency steadily rise.” Olivares added that if researchers find waves, their frequency could climb steadily as the two objects spiral toward collision.

Scientists think supermassive black holes exist at the centers of nearly all large galaxies and can weigh millions or even billions of times as much as the Sun, though scientists do not fully understand how they grow. Despite plentiful evidence of colliding galaxies, no one has reliably confirmed a close-orbiting pair of supermassive black holes.

Researchers consider supermassive black hole binaries the leading explanation for the gravitational wave background, and in 2023 the European Pulsar Timing Array and other collaborations reported evidence of this background. Pulsar timing arrays use the regular radio pulses of rapidly rotating neutron stars as precise cosmic clocks, measuring subtle changes in pulse arrival times to search for passing gravitational waves. The proposed Markarian 501 pair should produce low-frequency gravitational waves detectable with these arrays.

In June 2022, radiation from the system appeared to form an Einstein ring, a ringlike appearance produced when gravity bends light. The central black hole in Markarian 501 produces an intense jet of particles shooting into space at nearly the speed of light, driven by extreme conditions and powerful magnetic fields.

The full paper citation is “Detection of a second jet within the nuclear core of Mrk 501” by Britzen, Olivares and co-authors in Monthly Notices of the Royal Astronomical Society, 2026; 548 (4), DOI: 10.1093/mnras/stag291. This citation identifies the peer-reviewed paper underlying the claim. The ten authors include S Britzen, H Olivares, Gopal-Krishna, F Jaron, I N Pashchenko, E Kun, F K Schinzel, J Becerra González, D Paneque and N R MacDonald.