Sensitivity of the HAWC Observatory
The HAWC observatory is a second-generation facility based on the water Cherenkov technique pioneered at the Milagro observatory. Based on the experience of operating Milagro, the design of HAWC improves upon the performance of Milagro in several critical areas:
In order to observe gamma rays with high sensitivity, the HAWC data must be cleaned of cosmic-ray events. As described here, cosmic rays can be discriminated from gamma rays by observing the pattern of triggered PMTs in the detector (i.e., the "footprint" of the air shower). Gamma-ray showers tend to have a footprint that decreases radially from the center of the shower. In contrast, the footprints of cosmic-ray showers are relatively messy and will appear "blotchy" when observed in the pattern of triggered PMTs.
Using simulations of showers produced by gamma rays and cosmic rays, we have estimated our ability to reject cosmic-ray showers using triggered data from HAWC. Based on the PMT trigger patterns observed in the simulation, we find that we can reject >99% of cosmic-ray showers at energies above roughly 3 TeV using a simple topogical cut. Not surprisingly, the performance of the cut improves with energy as more and more information (PMT triggers) is present at higher energies. The removal of cosmic-ray events is a substantial improvement over Milagro, where 10% of the cosmic ray showers would survive in the data even for cuts at 50 TeV.
Almost as important as gamma/cosmic ray separation is the angular resolution of the detector. The angular resolution is defined as the typical error made when reconstructing the arrival direction of an air shower. All detectors have a finite angular resolution, which has the effect of smearing out features in observed sources. The smaller the angular resolution, the sharper the image will be.
It is advantageous to keep the angular resolution as small as possible. Obviously part of the advantage is being able to observe smaller structures; but the angular resolution also affects the sensitivity of the detector to gamma-ray point sources. This is because the point sources observed with any experiment will contain a mixture of "signal" (the events from the source) and "background" (the events not from the source). As the angular resolution decreases, the ratio of background to signal will drop. Essentially, the signal will stay constant — because it's a point source — while the background will decrease.
As shown in the figure above, the angular resolution of the HAWC observatory is about 0.1° for energies >10 TeV. It improves with energy because the number of PMTs hit increases with energy, in turn increasing the information available to the shower track fit. This resolution is a tremendous improvement over the Milagro detector, in which the best angular resolution was about 0.5°. It also means that the resolution of HAWC is similar to that of the very sensitive imaging air Cherenkov telescopes (IACTs) at the highest energies.
The energy resolution of HAWC refers to the typical error made when estimating the energy of the primary particle which initiated an air shower. A small energy resolution is an advantage because it allows for a more accurate measurement of the energy spectra of observed sources. If the energy resolution of a detector is poor, it is still possible to infer the spectrum of a source. However, this requires a very careful unfolding of the detector response from the physical spectrum, and this kind of procedure is prone to systematic error.
The expected energy resolution of HAWC has been calculated with simulated events. Above 10 TeV the energy resolution is below 50% — i.e., the energy of the particles observed above this threshold will by reconstructed to within 50% of the true energy. This is a major improvement over the energy resolution of Milagro, which was >100% for nearly all energies.