A stone falls into a pond, and ripples spread across the water. Likewise, light from distant galaxies spreads in all directions — calmly and steadily. But at the center of some galaxies, it's not a stone but a veritable cannon: it fires particles with monstrous force. Astronomers call such objects HSP BL Lac. These are active galactic nuclei where a supermassive black hole swirls matter into a disk and ejects narrow jets of plasma aimed almost directly at us.
In these jets, particles are accelerated so that their energy is millions of times their rest energy. Electrons spiral in the magnetic field and emit light. The frequency of this glow betrays their frantic energy — the peak falls in the ultraviolet and X-ray range (frequencies above 10 to the 15th power hertz). It's as if an ordinary lightbulb suddenly shone a million times brighter and shifted to rays invisible to the eye. These very objects are the prime suspects for producing ultra-high-energy cosmic rays and neutrinos with energies above hundreds of teraelectronvolts.
But the simplicity is only apparent. To produce cosmic rays with energies above 10 to the 19th power electronvolts (like the impact of a tennis ball compressed into a single subatomic particle), the jets must carry colossal energy, and in the form of heavy particles, not just light electrons. Calculations show that this would require too much "fuel." The energy budget of such objects is stretched to the breaking point. Moreover, the Pierre Auger Observatory sees more heavy nuclei than protons in the composition of the rays, and their arrival directions are almost uniformly distributed across the sky. This agrees poorly with rare "cannons" firing narrow beams. Perhaps the answer lies in magnetic fields that tangle the trajectories, or in jets that "breathe"—narrowing and widening, changing their aim.
New instruments (IceCube-Gen2, KM3NeT, the Cherenkov Telescope Array CTAO, the X-ray polarimeter IXPE, and upgraded cosmic-ray detectors) will allow us to measure key parameters. Where do neutrinos come from? How is the light of flares polarized? What exactly are the rays of extreme energies made of? The answers will show whether these cosmic cannons are truly the Universe's main accelerators, or whether the title goes to another class of objects. And then the ripples from distant galaxies will tell us far more than they do today.
The energy of a single ultra-high-energy proton is roughly equal to the kinetic energy of a flying mosquito, but compressed into a volume trillions of times smaller than an atomic nucleus.
In Carl Sagan's novel "Contact," the alien signal comes from the star Vega, but in the plot it is modulated by a neutrino stream — exactly the kind of particles that today are detected from blazars.