The High-Altitude Water Cherenkov Gamma-Ray Observatory

Discovering and Understanding Extreme Galactic Accelerators


Because the flux of gamma rays from all sources drops rapidly as a function of energy, observations of sources require a large effective area and long integration times, especially if the goal is to observe gamma rays above 10 TeV.

The point-source sensitivity of HAWC after 1 year of exposure is comparable to that of imaging air Cherenkov telescopes (IACTs) such as HESS and VERITAS after 50 hours of exposure. When comparing these two types of instruments, its important to keep in mind that IACTs have a much smaller field of view and can only operate on clear nights. In contrast, the HAWC detector has a large field of view and operates nearly 100% of the time. HAWC observes every gamma-ray source in half of the sky for 1500 hours every year. These two types of detectors complement each other allowing scientists to combine both deep and focused observation on individual sources with a synoptic survey 2/3 of the sky.

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Sensitivity of HAWC

Sensitivity vs. Energy
Point source sensitivity of the HAWC detector as a function of energy and integration time compared to other TeV observatories which use the imaging air Cherenkov technique. The Fermi-LAT instrument, which observes in the GeV band, is shown for reference.

Air Cherenkov telescopes typically observe sources for <50 hours, and survey observations last for about 10 hours. As seen in the figure above, at energies above roughly 6 TeV, the HAWC single-year sensitivity is better than the sensitivity of 50 hours of observation of a single source with VERITAS or HESS. An IACT can only spend up to about 200 hours per year observing a single interesting source due to ambient light and weather constraints on telescope operations. And above 10 TeV, the sensitivity of HAWC is better than the current generation of IACTs.

Currently, most of the HESS sources in the Galactic plane survey are not detected above 10 TeV. This energy is an important threshold because gamma-ray emission due to electron scattering of low-energy photons is expected to become inefficient at high energies. Sources with hard spectra above 10 TeV could be the best candidates for the acceleration sites of protons and other cosmic ray particles. The current generation of IACT would struggle to differentiate between different particle acceleration models like a hadronic gamma-ray spectrum vs. a leptonic spectrum with an exponential cutoff at 40 TeV. This kind of discrimination would be possible with HAWC. With 5 years of HAWC data, we will be able to measure 20 gamma rays above 100 TeV from a source with a spectral index of -2.3 and 20% of the flux of the Crab Nebula. Whether or not we see these 100 TeV gamma rays from sources will help us understand the acceleration processes and limitations at these sources.

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