Probing high energy particles in our Galaxy with multimessenger observations

In 1912, Victor Hess discovered that the Earth is bombarded by energetic particles originating from outer space. Since then, the origin, acceleration, and propagation of these cosmic-ray particles have remained as century-old questions. Cosmic rays are charged particles composed primarily of naked nuclei with energies millions of times greater than X-rays. Their deflection in magnetic fields and their interaction with matter and radiation fields throughout the Universe complicate studies of their sources. One promising approach is to study neutral particles, such as gamma rays and neutrinos, that are produced when cosmic rays interact. In this multimessenger approach, gamma rays and neutrinos preserve directional information, allowing us to directly observe the source sites where high-energy particles are accelerated. The rapid development of the field of gamma-ray astronomy, combined with the recent detection of a flux of astrophysical neutrinos, has opened new windows to study particle acceleration throughout the Universe.

I will highlight how combining the knowledge gained from measurements of cosmic rays, gamma rays, and neutrinos can broaden our perspectives and lead us toward a more complete picture of high-energy particles in our Galaxy. In particular, I will present what we have learned about the sources of high-energy particles with gamma-ray observations from the VERITAS experiment, a ground-based TeV gamma-ray observatory. I will also discuss what we expect to learn from future cosmic-ray measurements with HELIX (High Energy Light Isotope eXperiment), and future neutrino measurements with IceCube-Gen2.