The most powerful cosmic rays that we see on Earth may come from microquasars, small black holes that blast out jets of material from orbiting stars.

Unlike larger quasars, which feature a supermassive black hole, microquasars contain a black hole with a mass closer to that of the sun. The first discovered microquasar, SS 433, was found in 1975, but understanding the nature of its light-year spanning jets has been difficult because their structure looks remarkably different depending on which form of light you use to observe them.

When observed with lower-energy radiation, the jets start close to the centre of the black hole and appear to revolve like a spinning top. But using X-rays, or even higher-energy gamma rays, the jets appear to start very far from the black hole itself. Until recently, astronomers had only been able to get fuzzy pictures of them with gamma ray telescopes.

Now, at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, and her colleagues have observed the jets’ gamma radiation using the High Energy Stereoscopic System (HESS) telescopes in Namibia. They discovered that the jets’ high-energy structure changes over short distances, which suggests that the radiation is produced by electrons slamming into some kind of barrier and being accelerated.

If this process also occurs for heavier particles, such as atomic nuclei, then a microquasar closer to Earth operating in the same way could explain the mysterious, high-energy cosmic rays that we occasionally see colliding with our atmosphere, though so far the team can only definitively say that electrons are being accelerated.

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“This is really remarkable if you compare it to the way that accelerators are on Earth, because there you have huge machines that are super complicated, and they struggle to reach the highest [energy] values that we see out in the universe. And these systems are doing it somehow seamlessly,” says Olivera-Nieto.

“This is a beautiful reminder of the influence that black holes have way beyond their event horizons,” says at the University of Oxford. It is possible that heavier particles are being accelerated to create high-energy cosmic rays, but confirming this would require careful calculations and observations, she says.

Something is keeping cosmic rays out of the centre of the galaxy. The Milky Way is suffused with a sea of high-energy particles spewed out by supernovae and other energetic astrophysical objects, but that expanse of cosmic rays seems to be unexpectedly blocked out of the very middle of the galaxy.

at the Chinese Academy of Sciences and his colleagues examined data from the Fermi Gamma-ray Space Telescope on the distribution of cosmic rays near our galactic centre. They found that while the cosmic ray density was fairly constant in areas outside the very centre of the galaxy, it drastically dipped in the central molecular zone (CMZ), the area closest to the Milky Way’s middle.

This means that something is keeping the sea of cosmic rays out of the CMZ. “If there is no barrier, the cosmic ray sea component should also be present in the CMZ region,” says Huang. “However, the data indicate that it is just the opposite and a barrier must be present.”

Most cosmic rays are charged particles, so a sufficiently strong magnetic field would be able to change their paths. “It is likely that stronger magnetic fields exist in the CMZ than outside, which may block cosmic rays from penetrating into the CMZ,” says Huang. “In addition, there may be magnetised winds, driven by the activity of the central supermassive black hole Sagittarius A*, which also help stop particles from entering the CMZ.”

This is similar to a process that occurs in our own solar system, in which relatively low-energy cosmic rays are blown away by both the magnetised solar wind and the relatively strong magnetic fields throughout the system.

It could also occur on even larger scales, which could affect any extragalactic cosmic rays trying to get into the Milky Way, the researchers say.

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