The Milagro Gamma Ray Observatory at Fenton Hill, New Mexico, is designed to study "Very High Energy" (VHE) astronomical gamma rays and cosmic rays, for which a single particle carries about an erg of energy. Milagro is the first detector capable of continuously monitoring the full overhead sky at these energies. It complements the lower energy satellite experiments, higher energy air-shower experiments, and narrow field-of-view air-Cherenkov telescope experiments. A VHE gamma ray or cosmic ray interacting in the Earth's atmosphere initiates an extensive air shower, a cascade of interacting particles. The Milagro 22 million liter man-made pond is instrumented with detectors that measure light produced by those fast moving cascade particles that survive to detector altitude and enter the pond. Combining pond measurements with those by an array of detectors surrounding it allows determination of the energy and arrival direction of the primary particle that generated the cascade.
Since the energy of particles emitted by astronomical sources is determined by the physical processes occurring in the source, measurements at VHE energies, the highest at which astronomical sources have been observed, allows study of the most violent astronomical processes known. An example is gamma-ray bursts, currently believed to arise from either supernovae explosions or neutron star collisions. Active galactic nuclei, understood to contain massive black holes that swallow neighboring matter, are a detected source of VHE particles. Milagro has recently presented evidence for the first detection of VHE gamma rays emitted at the equator of our own Milky Way Galaxy. Milagro also detects solar coronal mass-ejection events, as an increase in detector rates, before they disrupt radio communication on Earth.
The particle energy and direction distribution generated by a source may be modified during the particles' interstellar or intergalactic path to Earth by interactions with background particles or bending of charged particle trajectories by magnetic fields. Measurement of VHE particles therefore also teaches us about the interstellar infrared radiation and sources of magnetic fields such as galactic clusters. The Solar magnetic field is studied by observing the shadowing of arriving VHE cosmic rays by the Moon and Sun.
Due to randomization of their arrival direction by galactic magnetic fields, cosmic rays arrive at the Earth nearly uniformly in direction. At VHE energies, however, small deviations are predicted due to the orbital motion of the solar system about the galactic center. Anisotropies observed by Milagro at the level of one part in a thousand are under study.
Searches for exotic VHE photon sources, such as weakly interacting massive particle (WIMP) annihilation or evaporation of primordial black holes, are also performed. As VHE sources without known lower energy counterparts have just begun to be discovered in the last few years, full-sky coverage is likely to provide additional surprises.
Cooperation between Milagro and various experiments at other energies allows symbiotic studies. The statistical power of the Milagro gamma ray burst analysis is enhanced when satellite detections are used as a trigger. Milagro will in turn be notifying the astrophysics community in real-time of any statistically significant signals it detects. Combined measurements by various experiments at different energies constrain models of particle production and interstellar background light absorption. Similar advantages exist when Milagro measurements of coronal solar mass-ejection events are combined with those of neutron monitors.
