At night, invisible bullets streak across the sky — cosmic rays. These are atomic nuclei accelerated to near-light speeds. The Galaxy's magnetic fields tangle their paths, making it hard to pinpoint where they were born. Supernova remnants have long been the prime suspects, but for the fastest particles, the evidence was lacking.
Now, black holes paired with ordinary stars take the stage. When a black hole siphons matter from its companion, it doesn't always swallow everything — sometimes it flings some back out, like a laundress wringing water from laundry. Such outflows plow through space, sweeping up the tenuous gas ahead of them. At the boundary between the outflow and the gas, particles can be accelerated, like surfers riding a steep wave.
The scientists calculated that in such 'waves,' particles can gain energies up to 20 million teraelectronvolts. That's roughly like a grain of sand accelerated to the energy of a speeding truck. The contribution of all such outflows to the Galaxy's total cosmic ray flux is about one percent, but for the most energetic particles, it rises to five percent.
Future telescopes, such as CTAO, will be able to test these calculations by catching gamma-ray flares and neutrinos from outflows. If everything adds up, black hole spin-dryers turn out to be a key part of the cosmic kitchen where the fastest particles in the Universe are cooked up.
The energy of one such particle can be millions of times greater than that of particles in the most powerful accelerator on Earth, but it can only be detected indirectly — through flashes of light and neutrinos.
In Stanisław Lem's novel 'Solaris,' the planet's ocean spawns mysterious structures — somewhat like how a black hole ejects matter that later accelerates particles.