Tag: Astronomy

Astronomers have found evidence of a second, previously hidden jet at the center of a galaxy called Markarian 501 — a sign, they argue, that the galaxy’s known supermassive black hole has a close companion, locked in an orbit lasting only about four months. If that binary interpretation holds up, the pair would be among the closest candidate supermassive black hole pairs ever identified, and under some assumptions could merge within roughly a century, giving astronomers an unusually good opportunity to track the late stages of a process that shapes how galaxies grow.

A Jet That Shouldn’t Be There

Markarian 501, a galaxy about 450 million light-years away in the constellation Hercules, has been a familiar target for radio astronomers for decades. It belongs to a class of galaxies called blazars, whose central black hole fires a narrow jet of energetic particles and radiation almost directly at Earth, making the galaxy unusually bright at radio and gamma-ray wavelengths. That single jet is what astronomers had always used to study the galaxy’s core.

A team led by Silke Britzen at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, re-examined archival radio images and found a second, fainter jet-like feature, appearing on the opposite side of the core from the known jet and curving around it from image to image. Because it was faint and positioned differently from the familiar jet, it had gone unremarked in earlier studies of the galaxy.

Twelve Years of Radio Snapshots

To check whether the second jet was real rather than an imaging artifact, the team reanalyzed 83 Very Long Baseline Array (VLBA) observations at 43 gigahertz, taken between September 2011 and July 2023 — a US-wide network of radio dishes that, combined, works like a single extremely high-resolution telescope. They compared these with earlier Mrk 501 observations at 15 and 8 gigahertz, extending the galaxy’s radio monitoring history back further still.

Across the 83 snapshots, the second feature kept reappearing in a shifted position, tracing a path that looked like it was sweeping around the core. Separately, the brightness of the galaxy’s central core rose and fell in a recurring pattern. Neither behavior is expected from a single, stable jet; both become easier to explain if a second massive object is orbiting the first and periodically disturbing the flow of material near the black hole.

A Partial Ring in a 2022 Image

One image, taken on June 24, 2022, offered an especially intriguing clue: a partial ring-like structure around the core, which the researchers interpret as a possible Einstein ring, a pattern that can appear when light from a background source is bent into a circle by the gravity of a massive foreground object. In this case, the proposed explanation is that light from the second jet was briefly bent around the main black hole as the two objects passed close to the same line of sight.

The team treats the ring as supporting evidence rather than confirmation. The two candidate black holes sit far too close together, as seen from Earth, for any existing telescope — including the Event Horizon Telescope — to resolve them as two separate points of light, so no image has shown them individually side by side.

A Four-Month Orbit, Not Perfectly Pinned Down

The two independent clues point to a similar, short rhythm, but not an identical one. Comparing images in which the second feature returned to a similar position gave a recurrence time of about 146 days, with sizeable uncertainty of plus or minus 22 days. The separate pattern in the core’s brightness pointed to a period closer to 121 days, though the researchers describe that figure as less securely measured. The paper adopts roughly 121 days as its working estimate for the orbital period, while noting the two measurements do not perfectly agree.

For two supermassive black holes to complete an orbit in only about four months, they would have to be extraordinarily close together: for objects with roughly equal mass, the team estimates a separation of only 27 to 128 times the black hole’s own Schwarzschild radius — the characteristic size of a black hole’s event horizon — assuming each black hole weighs somewhere between 100 million and 1 billion times the Sun’s mass. Converted to more familiar units, the Max Planck Institute for Radio Astronomy’s own announcement describes this as roughly 250 to 540 times the distance between Earth and the Sun, depending on the exact masses assumed.

That is a strikingly small gap for two objects this massive. Many galaxies are thought to harbor black hole pairs left over from earlier galaxy mergers, which sink toward each other over millions of years but then stall at a particular separation — a long-standing puzzle known as the “final-parsec problem,” since theory alone has struggled to explain how such pairs lose enough orbital energy to close that last stretch of distance. A confirmed pair this close together would offer scientists a rare, concrete look at what that advanced stage actually looks like.

A Possible Merger in About a Century

Based on that tight separation, the researchers estimate that the binary’s orbit could shrink enough for the two black holes to merge on a timescale as short as roughly 100 years, under certain assumptions about their masses and mass ratio. That is not a firm countdown: it is a model-dependent estimate, and if the true masses sit toward the heavier end of the estimated range, the inferred merger timescale lengthens considerably.

The researchers are careful to frame the whole result as an interpretation, not a settled fact. The paper’s language throughout is “appears to,” “is consistent with,” and “likely,” rather than “proven,” and it does not resolve the mismatch between the 121-day and 146-day period estimates. What is observed directly is the second jet-like feature, its shifting position, the brightness cycle, and the one-time ring; the orbiting-black-hole picture is the explanation the authors find best fits all of it together.

A Chance to Track, Not Necessarily Witness

If the binary interpretation is confirmed, Markarian 501 would become a valuable target for a very different kind of observation: gravitational waves. As two supermassive black holes spiral closer together, they are expected to emit extremely low-frequency gravitational waves — ripples in spacetime far too slow for ground-based detectors like LIGO, but potentially detectable by pulsar timing arrays (PTAs), which track tiny timing shifts in the ultra-regular radio pulses from rapidly spinning dead stars called pulsars.

In 2023, several PTA collaborations, including the European Pulsar Timing Array, reported the first evidence of a faint background hum of gravitational waves, widely thought to come from the combined signal of many such black hole pairs across the universe, blurred together. Picking out a single, confirmed, well-characterized pair like the one proposed in Markarian 501 from that noisy background would be a demanding task, but the researchers see it as a genuine long-term possibility.

“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.

That prospect is what makes this finding more than a curiosity about one distant galaxy. Supermassive black hole mergers are thought to be common over cosmic history, but astronomers have never had a well-measured candidate this close to the final stage. Whether or not Markarian 501’s pair turns out to be real, the search for it is already pushing researchers to track changes in its jets, brightness, and possible gravitational-wave signal more closely, over the human-scale years and decades ahead.

Sources: Max Planck Society, Max Planck Institute for Radio Astronomy, Britzen et al. 2026, Monthly Notices of the Royal Astronomical Society