The High-Altitude Water Cherenkov Gamma-Ray Observatory

Astronomy at the TeV Scale

Gamma Ray Physics

How and where are cosmic rays accelerated? This 100-year-old question drives the field of high-energy particle astrophysics.

A map of the galactic and extragalactic γ-rays observed by the Fermi Gamma-ray Space Telescope (2011).

Cosmic rays are charged particles. We believe they are accelerated in tremendous astrophysical explosions such as supernovae, gamma-ray bursts, and the turbulent regions of space near supermassive black holes. By studying cosmic rays, we hope to gain a better understanding of these violent (and ubiquitous) objects.

High-energy gamma-ray observations are an essential tool in the study of the origins of cosmic rays, because gamma rays are created when cosmic rays interact with material near their acceleration sites. Because they are electrically neutral, the gamma rays produced in such interactions are not perturbed by the magnetic fields which fill our own galaxy and intergalactic space. Therefore, we can use them to perform gamma-ray astronomy.

By observing the spatial distribution and intensity of gamma rays across the sky, we can attempt to identify the locations of cosmic ray accelerators. In addition, the time variability and energy spectra of the gamma-ray emission can be used to study the environment of the accelerators and the mechanisms of charged-particle acceleration. The highest-energy gamma rays (above 1 TeV) and the shortest timescales of variability provide important constraints on the mechanisms at work in acceleration sites.

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Cosmic Ray Physics

For gamma-ray physics, cosmic-ray air showers are the source of a large background that is only removed with some difficulty. However, at TeV energies the cosmic rays are themselves a fascinating topic for study.

An equatorial coordinate map of cosmic ray "hot spots" in the Milagro and IceCube data. The solid line is the galactic plane; the dot is the galactic center.

Since 2007, several gamma-ray observatories located in the northern hemisphere (including Milagro) have reported an anisotropy in the arrival directions of the cosmic rays around 10 TeV. Although the anisotropy is quite small, with an amplitude of a few parts per thousand, its presence is still surprising, because it is expected that galactic magnetic fields should completely randomize the arrival directions of cosmic rays in this energy range.

In 2011, cosmic ray measurements from the IceCube Neutrino Observatory demonstrated the extension of the anisotropy to the southern hemisphere. The origin of these features — a nearby supernova remnant? heliospheric effects? magnetic lensing? — are not known. The HAWC Observatory, which has a large field of view, is in an excellent position to follow up on these interesting studies.

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