Speed or illusion? The mystery of accelerating black hole jets

The universe is full of powerful forces, but few are as mysterious as the high-speed jets that blast out from supermassive black holes.

These plasma jets, emerging from the centers of certain galaxies known as active galactic nuclei (AGN), shoot across space at nearly the speed of light.

Scientists have long sought to understand how they form, why they accelerate, and what role magnetic fields play in shaping them.

A new study led by experts from the Max Planck Institute for Radio Astronomy (MPIfR) in Germany and the Instituto de Astrofísica de Andalucía (IAA-CSIC) in Spain is shedding light on these questions.

Using detailed radio observations of 16 different AGN, the research team looked deeper than ever before – right up to the region near the black hole where these powerful jets originate.

Global network of radio telescopes

The team relied on data collected during the 2017 campaign of the Event Horizon Telescope (EHT), a global network of radio telescopes that together form a virtual Earth-sized observatory.

Known for capturing the first-ever image of a black hole in the galaxy M87, the EHT offers unmatched resolution. It’s capable of resolving structures just outside the black hole’s event horizon.

In this study, researchers compared the EHT’s ultra-high-resolution observations with previous images taken by the Very Long Baseline Array and the Global Millimeter VLBI Array. These older observations captured the jets at much larger scales.

By combining data across different frequencies and resolutions, the scientists pieced together a more complete picture of how these jets evolve from their starting point near the black hole to the vast stretches of interstellar space.

Zooming in on black hole jets

“Our findings challenge long-standing assumptions about how jets behave,” said Jan Röder, who led the research team.

“By analyzing a sample of sixteen active galactic nuclei, we were able to reduce the influence of individual peculiarities and obtain a broader picture of jet behavior.”

Traditionally, jets were thought to follow a simple pattern. The most common model describes them as cone-shaped, with plasma moving at a constant speed while the magnetic field and plasma density decrease with distance.

Astronomers would expect to predict the appearance and behavior of jets based on this model, but the new findings indicate otherwise.

Broader picture of jet behavior

Röder noted that the basic model cannot be a perfect description for all jets – most likely, only for a small fraction. “The dynamics and sub-structure of jets are intricate, and observational results can suffer greatly from astrophysical degeneracies.”

“For example, we know that many jets appear to accelerate. Either the plasma itself accelerates, or it can be an effect of geometry: if the jet bends, it may point at us more directly, giving the impression of faster movement,” said Röder.

Project co-leader Maciek Wielgus from IAA-CSIC explained that by using a sample of sixteen active galactic nuclei, the team was able to get a broader picture of the behavior of jets, compared to just looking at individual sources. “This way, the results are less prone to influence from their respective unique properties.”

The experts also noticed something interesting about jet brightness. As the plasma moved farther from the black hole, its brightness typically increased. This suggests that the jet may be accelerating, or that some other energy-related process is taking place as it travels outward.

Tracking changes in black hole jets

One key to understanding these jets lies in the “middle ground” – the space between what the EHT sees and what older arrays reveal.

“The Global Millimeter VLBI Array operating at 3.5 mm wavelength provides the key information between the highest resolutions achieved by the EHT and the more general picture of jets provided by the Very Long Baseline Array,” noted Eduardo Ros, the European Scheduler of the GMVA.

“This was evident in the case of M87, as presented by RuSen Lu and collaborators in April 2023.”

These overlapping observations allow scientists to track how black hole jets change across distance and time. They also help identify where existing models fall short – especially when it comes to how jets accelerate or interact with their surroundings.

Understanding the acceleration mechanism

Even with this new data, some mysteries remain. The idea that brightness increases with distance contradicts older theories and opens the door for new ones. While this could be due to changes in jet structure, more research is needed.

“More studies are needed to fully understand the acceleration mechanism, the flow of energy, the role of magnetic fields in jets of active galactic nuclei, and their geometries,” said Röder.

He noted that the expanding EHT array will play an important role in the future discoveries on these fascinating objects. Collaboration will continue to play a crucial role as well.

“These results are based on the ongoing work of the EHT and are confirmed by the Global Millimeter VLBI Array studies. They demonstrate the importance of global partnerships, cutting-edge technologies and persistent research for scientific progress,” said J. Anton Zensus, director at MPIfR and founding chair of the EHT collaboration.

“With new telescopes and the next generation of networks, we will continue to deepen our understanding of these fascinating cosmic phenomena.”

The full study was published in the journal Astronomy & Astrophysics.

Image Credit: NASA/JPL-Caltech

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