Illustration: Alan Stonebraker

Figure 1: (a) When high-energy cosmic rays or photons enter the atmosphere, they initiate a cascade of millions of secondary particles that is called an extensive air shower. (Source: Milagro collaboration [9].) (b) Milagro is an underwater array that detects such secondary air shower particles by the Cherenkov effect. The observations allow researchers to reconstruct the energy and direction of the primary particle that had initiated the air shower. (Photograph by Rick Dingus, Texas Tech University.) (c) Distribution of cosmic rays in the sky including regions of excess TeV cosmic rays (A, B). The black dot shows the direction of the heliotail, the region opposite the direction in which the solar system moves through the interstellar medium, which may be the source of the cosmic-ray excess. The color bar shows significance of the detected excess in $σ$ units. (Adapted from Abdo et al. [2].)