Two supermassive black holes in the galaxy Mrk 501 could collide in as little as 100 years

Two supermassive black holes in the galaxy Mrk 501 may be locked in a tight orbit and could merge in as little as 100 years, according to a team led by Silke Britzen of the Max Planck Institute for Radio Astronomy in Bonn, Germany. The claim rests on the detection of a second jet that astronomers had been hunting for a long time.

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Mrk 501 is already famous among radio astronomers because its known central black hole fires a jet almost straight toward Earth, which makes the object bright and easy to track. A second black hole is far harder to catch. Its jet points in a different direction, so it does not shine into the telescopes the way the first one does, and it can hide behind its much more conspicuous neighbor for years at a stretch.

How a second jet gave the companion away

The evidence comes from radio observations made at several frequencies on dozens of separate days across roughly 23 years, according to the ScienceDaily report on the study. Over a span of a few weeks, a faint jet appeared to emerge from behind the larger black hole and sweep counterclockwise around it. Britzen said it came as a complete surprise that the team could not only see a second jet but follow its motion, and she described evaluating the data as feeling like being on a ship.

The black holes themselves cannot be resolved. The Max Planck Society press release notes that the distance is so great that even the Event Horizon Telescope cannot separate the two objects, so the second hole is inferred from its jet and not seen directly. That inference matters for everything that follows, because each number about the pair is derived from how the jets move and brighten rather than measured from the holes.

What the orbit and mass estimates imply

From the jets’ changing positions and recurring brightness patterns, the team estimates that the pair may orbit each other every 121 days. Their separation is put at roughly 250 to 540 times the distance between the Earth and the Sun, which is tight by the standards of galaxy-scale objects. The masses are estimated at about 100 million to 1 billion times the mass of the Sun, and the ranges are wide enough that the final answer depends on where in them the real values fall.

That is where the 100-year timescale comes from. Heavier black holes radiate gravitational waves faster and spiral together sooner, so the merger time scales with mass. If the masses sit toward the high end, the researchers say the merger could come in as little as 100 years; lighter masses would stretch the wait. Because the figure is a lower bound tied to an uncertain input, it is better read as a possibility than a countdown.

One more observation supports the picture. On a single day in June 2022, light from the second jet appears to have been bent into a ring by the gravity of the foreground black hole, an effect called an Einstein ring. According to the press release, such an arrangement would require the system to line up almost perfectly with Earth, and the team presents it as one interpretation consistent with a binary rather than a confirmed proof.

Pulsar timing arrays and the test ahead

A close pair of this size should emit extremely low-frequency gravitational waves, and the researchers point to pulsar timing arrays as the instruments that could in principle detect them. The ScienceDaily account recalls that the European Pulsar Timing Array and other collaborations reported evidence in 2023 of a gravitational-wave background that such binaries are thought to feed. Mrk 501 is described as a prime candidate for a source such a network could pick out, though no detection has been reported.

Co-author Héctor Olivares said that if gravitational waves are detected, astronomers may even see their frequency steadily rise as the two giants spiral toward collision. A signal like that would offer a rare chance to watch a supermassive black hole merger unfold, and it would give an independent check on the orbit and masses derived from the jets.

The work, titled “Detection of a second jet within the nuclear core of Mrk 501,” has a journal record in Monthly Notices of the Royal Astronomical Society, where it is listed for volume 548, issue 4. The ScienceDaily page describes the paper as accepted for an upcoming issue, so the details may be refined. The institute’s own Bonn research page is the place to look for follow-up announcements. The authors also include Gopal-Krishna, F. Jaron, E. Kun and F. K. Schinzel among others.

Supermassive pairs this close are rare finds. An earlier ScienceDaily report on the closest pair of supermassive black holes yet shows how few candidates have been put forward. If the Mrk 501 interpretation holds up, it would give astronomers a nearby laboratory for how such systems shrink, with the caveat that a century is a tiny span on cosmic scales and the real merger could be far later.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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