Astronomers Image the First Close Binary Supermassive Black Hole Pair, Caught Spiraling Toward a Merger in Markarian 501
A 23-year radio campaign on the blazar Mrk 501 reveals a second jet from a hidden supermassive companion orbiting every 121 days, the first directly imaged close SMBH binary.
Overview
A team led by Silke Britzen of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn has reported the first direct detection of two supermassive black holes orbiting one another at close range, hidden inside the well-known blazar Markarian 501. The result, announced by MPIfR on April 7, 2026 and accepted for publication in Monthly Notices of the Royal Astronomical Society, rests on roughly 23 years of high-resolution radio imaging that revealed an unexpected second relativistic jet curling around the primary on a 121-day cycle.
If the interpretation holds, Mrk 501 becomes the first close-pair supermassive black hole (SMBH) system ever resolved, and a prime target for the pulsar timing arrays now hunting nanohertz gravitational waves.
What We Know
Markarian 501 is an elliptical galaxy and BL Lac-type blazar in the constellation Hercules, lying more than 450 million light-years from Earth. Astronomers have monitored its core for decades because its relativistic jet points almost directly at us, making it one of the brightest extragalactic sources of very-high-energy gamma rays.
What changed is the imagery. Britzen’s team combined dozens of multi-frequency radio sessions over roughly two decades to track structure on scales much smaller than a single jet feature. As phys.org reported, the long baseline of observations exposed not just one jet but a second, fainter outflow whose appearance shifted within a few weeks and traced out a closed orbital path.
From the motion of that second jet, the team derived the system’s basic parameters. According to the MPIfR announcement, each black hole carries between 100 million and 1 billion solar masses, the two are separated by roughly 250 to 540 astronomical units, and the secondary completes one orbit every 121 days. Universe Today’s coverage characterizes the pair as “two objects each containing hundreds of millions of solar masses, circling each other in a tightening orbit.”
Britzen, in comments distributed with the press release, described the find as a long-sought target that surfaced unexpectedly in the data: “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.”
The gravitational-wave implications are central to why the result matters. As phys.org notes, at this separation and these masses the inspiral should radiate gravitational waves in the nanohertz band, the regime that international pulsar timing arrays (PTAs) are designed to probe. PTAs reported a tentative stochastic gravitational-wave background in 2023, but the signal could not be assigned to any specific source. The MPIfR team argues that Mrk 501 is now a leading candidate for attributing some of that emission to an identified binary.
The team’s models suggest the two black holes could merge in roughly 100 years, with the exact timeline depending on their true masses, according to phys.org’s reporting. On cosmological scales that is essentially instantaneous; on human ones, it places Mrk 501 in an extraordinarily late phase of inspiral.
What We Don’t Know
The central caveat is that the second black hole is inferred, not seen. Even the Event Horizon Telescope cannot resolve two objects this close at this distance, as Universe Today notes, so the case rests on the dynamics of the second jet rather than a direct image of two compact sources. Alternative explanations — jet precession from a single black hole, internal magnetohydrodynamic instabilities, or warped accretion disks — have been invoked in the past for similar quasi-periodic blazar behavior, and independent groups will need to test whether the orbital model fits the multi-decade dataset better than these single-engine scenarios.
The individual masses are also bracketed by an order of magnitude, from 100 million to 1 billion solar masses each, per the MPIfR figures. That uncertainty propagates directly into the gravitational-wave amplitude predicted for pulsar timing arrays and into the merger timescale, meaning the “100 years” estimate is a best-case-of-several rather than a fixed forecast.
It is also unclear how representative Mrk 501 is. Galaxy mergers should produce SMBH binaries routinely, yet none had been spatially resolved at sub-parsec separations before this work. Whether Mrk 501 is the first of many such detections, or an unusually favorable orientation that made the second jet visible, will depend on similar long-baseline radio campaigns being applied to other candidate systems.
Why It Matters
This discovery sits at the intersection of two long-running observational programs that have so far been largely separate. Pulsar timing arrays have been narrowing in on the nanohertz gravitational-wave background for years, while VLBI radio astronomy has been resolving blazar cores at ever finer scales. If Mrk 501 truly hosts a binary on a 121-day orbit, the same object can in principle be tracked simultaneously in radio and in gravitational waves — a multi-messenger handle on supermassive black hole physics that has not been available before.
For now, the MPIfR result opens a target list of one. If pulsar timing arrays can match a coherent signal to that orbit in the coming years, the two fields will have linked a specific galaxy to a specific gravitational-wave source for the first time.