Supermassive black holes at the centres of galaxies emit radiation and ultra-fast winds into space. Here is an artistic interpretation. Illustration: NASA, JPL-Caltech. Public Domain.

The Universe is full of cosmic radiation but why it can be of a much higher energy than usual has remained a mystery for over 60 years.

This cosmic radiation consists of –- produced by black holes, gigantic supernovae, or rotating neutron stars – a type of dead star. These particles contain energy.

Sometimes, however, cosmic rays have a much higher energy than usual. We have known about this since 1962, but we still don’t know why. We also don’t know where this ultra-high-energy cosmic radiation comes from. Or do we?

Supermassive black holes may be the cause

Foteini Oikonomou is working on the case. She is an associate professor at the Norwegian University of Science and Technology‘s (NTNU) Department of Physics.

In a newly published article, she and her colleagues present a completely new and plausible explanation: that this high-energy radiation is created by winds from supermassive black holes. But what on earth does that mean?

Active black holes create winds

The Milky Way is our neighbourhood in the Universe. Our Sun and the Solar System are part of this galaxy, along with at least 100 billion other stars.

postdoctoral fellow Enrico Peretti from the Université Paris Cité, said: “There is a black hole called Sagittarius-A* located right in the centre of the Milky Way.

“This black hole is currently in a quiet phase where it isn’t consuming any stars, as there is not enough matter in the vicinity.”

This contrasts with growing, supermassive, active black holes that consume up to several times the mass of our own Sun each year.

Peretti explained: “A tiny portion of the matter can be pushed away by the force of the black hole before it is pulled in.

“As a result, around half of these supermassive black holes create winds that move through the Universe at up to half the speed of light.”

We have known about these gigantic winds for approximately ten years, and by blowing away gases, they can affect the surrounding galaxies and prevent new stars from forming.

This is dramatic enough in itself, but Oikonomou and her colleagues looked at something else, much smaller, that these winds could be the cause of.

Lead author and NTNU PhD research fellow Domenik Ehlert, said: “It is possible that these powerful winds accelerate the particles that create the ultra-high-energy radiation.”

To understand this, we also need to explain a little bit about atoms.

Atoms and enormous amounts of energy

Atoms consist of a nucleus, which is made up of protons and neutrons. These particles are made up of quarks, but we don’t need to go into that right now.

One or more electrons can be found around this nucleus in the so-called cloud.

Oikonomou said: “The ultra-high-energy radiation consists of protons or atomic nuclei with energy up to 1,020 electron volts.”

A particle like this, which is smaller than an atom, contains about as much energy as a tennis ball when Serena Williams serves it at 200 kilometres per hour.

If that number doesn’t mean anything to you, you should know that in this context, it is an absolutely enormous amount of energy.

It corresponds to approximately a billion times more energy than the particles created by researchers in the Large Hadron Collider in Switzerland and France.

Fortunately, these cosmic rays are destroyed by the Earth’s atmosphere. When they reach ground level, they are as harmless as all the other cosmic radiation we receive.

Oikonomou said: “But for astronauts, cosmic radiation is a very serious problem.

“The main concern for astronauts is cosmic low-energy radiation produced by our own Sun, because it is much more common. The rays we study are infrequent enough that it is extremely unlikely they would pass through an astronaut.”

Other suspects

Previously, researchers have looked into whether these high-energy particles come from gamma-ray bursts, from galaxies that are creating new stars at an extremely high rate, or from plasma outflows from supermassive black holes.

However, Oikonomou and her colleagues have another hypothesis.

Ehlert said: “All the other hypotheses are very good guesses – they are all sources that contain a lot of energy. But no one has provided evidence that any of them are the origin.

“That is why we decided to investigate the winds from the supermassive black holes.”

Guilty? Maybe

So what do we actually know? Is it the winds that create the high-energy particles in the cosmic radiation?

When researchers ask questions like this, they often feel a sense of excitement and think “YES, that might just be the case!”

Oikonomou said: “Our answer is more of a cautious ‘maybe’. We find that the conditions related to these winds align particularly well with particle acceleration.

“But we are still unable to prove that it is specifically these winds that accelerate the particles behind the high-energy cosmic radiation.”

However, the model they are using can explain one specific aspect of these particles that we still don’t understand. Within a certain energy range, the particles have a chemical composition that other models cannot explain in any meaningful way.

Oikonomou concluded: “We can also test the model using neutrino experiments. In the years to come, we hope to collaborate with neutrino astronomers to test our hypothesis.”

The study is published in Monthly Notices of the Royal Astronomical Society.