At the centers of many galaxies lurk giant black holes. They are surrounded by dense clouds of hot plasma that emit a powerful X-ray glow. In a cramped region, just a few gravitational radii (the radii of Karl Schwarzschild — the boundary beyond which even light disappears), protons are accelerated to monstrous energies. This is the entrance to nature’s factory.
When an accelerated proton collides with an X-ray photon, a reaction kicks off: $p + \gamma \rightarrow \Delta \rightarrow n + \pi^+$. Here’s how it plays out: the short-lived delta particle decays into a neutron and a pion, and the pion splits into a neutrino and a muon. Neutrinos fly outward, carrying energies of tens of teraelectronvolts (that’s trillions of times more energetic than visible light particles). Meanwhile, gamma rays get absorbed by the dense plasma, and from the outside, the source appears silent. Neutrinos become the factory’s only voice.
Data from the IceCube neutrino telescope, built deep in the Antarctic ice, paint a clear picture. The captured neutrino flux turned out to be directly proportional to the X-ray brightness from a compact region no larger than ten radii of Karl Schwarzschild. This allows us to confidently say: the main sources of such particles are the active cores of galaxies, where black holes grind the energy of infalling matter into directed beams. Most likely, these hidden factories create the very diffuse background of neutrinos and extragalactic cosmic rays that has puzzled scientists for decades. The conveyor belt runs nonstop.
Neutrinos are so elusive that it would take a lead slab a light-year thick to guarantee stopping just one.