Gamma rays and neutrinos as clues to the origin of high energy cosmic rays.

Compact regions in the Milky Way, such as accreting degenerate binary stars, may be sites of acceleration of particles with energies far greater than produced at any man-made accelerator, present or proposed. If so, they would emit characteristic neutral radiation of ultra-high energy, which might be strong enough to be detectable at Earth. The quest for these faint but energetic signals is the focus of more than 50 large, ground-based experiments that are looking for high energy photons or neutrinos from point sources in our galaxy and beyond. Several sources have been claimed, but the signals appear to have unexpected and puzzling features that must be clarified before the field can settle into a routine phase of systematic investigation. In the meantime, the potentially profound implications for particle physics, as well as astrophysics, make this field one of intense activity.