A stream of water from a hose hitting a wall splashes sideways. Astrophysical jets behave similarly — narrow plasma flows ejected from the vicinity of black holes or young stars. When such a flow encounters resistance from surrounding gas, a shock wave forms inside it, akin to a standing wave in a fast-flowing river. At this wave, charged particles, like balls in a game of ping-pong, bounce repeatedly and gain energy.
Scientists have built a model that shows where in the jet such a shock wave arises and how strongly it accelerates particles. The model was applied to three types of objects: Seyfert galaxies (active nuclei with moderate brightness), microquasars (binary systems with a stellar-mass black hole), and protostellar jets (outflows from forming stars). It turns out that in Seyfert galaxies, protons can be accelerated to energies of billions of teraelectronvolts — millions of times higher than at the Large Hadron Collider.
These results help us understand where the most energetic particles in the universe come from. Perhaps such shock waves in jets are responsible for ultra-high-energy cosmic rays detected by ground-based observatories. They might also explain the mysterious gamma-ray emission from some galaxies.
The energy of a proton accelerated in a Seyfert galaxy's jet is comparable to that of a flying tennis ball, but concentrated in a single subatomic particle.