The High-Altitude Water Cherenkov Gamma-Ray Observatory

Galactic Cosmic Rays

Open Questions

X-ray observations of the Tycho SNR
X-ray observations of stellar debris (green/red) and filamentary shells of high-energy electrons (blue) in Tycho's supernova remnant. Such objects are believed to be the source of cosmic rays in the Galaxy. From NASA/CXC/Rutgers/J.Warren & J.Hughes et al.

Cosmic rays up to at least 103 TeV, and perhaps as high as 106 TeV, are thought to be of Galactic origin. The observed spectral break at 103 TeV known as the "knee" may be due to the spectrum of cosmic ray sources, the escape of the cosmic rays from the Galaxy, or a combination of the two effects. It has even been speculated that a single nearby source is the cause of the knee.

When protons with energies of about 103 TeV collide with molecular clouds or other matter the resulting interactions will produce ~100 TeV gamma rays. These very high-energy gamma rays will be observable by HAWC from individual point and extended sources as well as from the sea of cosmic rays interacting with matter in the Galactic plane.

Supernova remnants (SNRs) have been postulated as the origin of Galactic cosmic rays largely because they have sufficient energy to provide the observed local cosmic ray energy density. SNRs also have sufficiently strong magnetic fields to trap particles long enough to accelerate them up to at least 100 TeV. TeV gamma rays have been observed from SNRs, but also from other Galactic sources, such as pulsar wind nebulae and compact binary systems. Which of these sources accelerate hadronic cosmic rays? What is the total power output of these Galactic accelerators? Observations of many different types of sources at the very highest energies are essential to answer these questions. up to the highest energies of many sources from different classes are essential to answer these questions.

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The Origins of TeV Emission

Gamma-ray observations at the highest possible energies are key to distinguishing gamma rays produced by electrons from those produced by hadrons. There are observational differences in the TeV gamma-ray spectrum from electron accelerators and proton accelerators accessible to HAWC. Electrons lose their energy more quickly than protons due to synchrotron emission and are therefore more difficult to accelerate to the highest energies. Also, the cross section for inverse Compton scattering decreases at higher energies, resulting in a break in the gamma-ray spectrum by at least 10-50 TeV. Gamma rays from hadronic cascades in the accelerating region, on the other hand, follow the power law spectrum of the particles initiating the cascades up to the highest energies.

Therefore, a primary goal of HAWC is to perform careful measurements of the fluxes of TeV sources up to 100 TeV. Sources with very hard spectral indices that don't exhibit sharp cutoffs in the 10 TeV range are good candidates for hadronic acceleration.

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