Mysterious black hole jets may be the source of powerful cosmic rays

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, Laura Olivera-Nieto 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.

“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 Katherine Blundell 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.