Gamma-ray/Neutrino production
The cosmic-rays detected in the Earth atmosphere find their origin in astronomical sources of high energy (see cosmic explosions). Shocks and possibly magnetic reconnection result in the acceleration of particles to very high energies, with the the distribution of energies being non-thermal, often following quasi power-law distribution.
Apart from detecting them as cosmic rays on Earth, these relativistic particles can also be studied in the highly energetic sources where they are produced, and in the interstellar medium through which they may subsequently propagate, through the electro-magnetic and neutrino signals they produce.
The best studied electro-magnetic signal is synchrotron radiation from relativistic electrons and positrons interacting with magnetic fields. Synchrotron radiation is one of the most important signals in the radio domain, and can be studied with radio-telescopes such as, in the Netherlands, the Westerbork radio-synthesis telescope and LOFAR. The highest energy electrons/positrons produce synchrotron radiation in X-rays.
The same electron/positron populations that produce synchrotron radiation also produce signals with higher frequencies. The relativistic electrons can also scatter background photons, which then gain energy, and can be detected as gamma-rays. The background photons are usually part of the cosmic microwave background, but could also find its origin stellar light, or even come from the source itself. Electrons/Positrons also produce gamma-rays by interacting with charged particles, a process called bremsstrahlung (or free-free emission).
The cosmic rays observed on Earth consist for 99% of protons and other atomic nuclei. Also these produces signals: collisions of these particles with background atoms leads to the production of new particles, in particular so-called pions. These pions come in three flavors π+, π-, π0. The π0 decays almost immediately in two photons, which we can observe as gamma-rays with energies in excess of 100 MeV. The charged pions (π+, π-) decay into muons and neutrinos, with the muons decaying into electrons/positrons and additional neutrinos. The electrons/positrons will in principle produce radiation as well, but of interests are also the neutrinos, as they can be detected by giant neutrino detectors such as KM3NeT and IceCube.
The combination of all these signals (radio, X-ray, gamma-rays and neutrinos) can be used to study the sources of cosmic rays, and the physics of the acceleration process.
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