HAWC: The High Altitude Water Cherenkov Experiment

High energy gamma rays probe the most extreme astrophysical environments including those that produce the highest energy cosmic-ray particles. The Milagro observatory has demonstrated that a detector with a wide field of view (2sr) and nearly 100% duty cycle can discover new sources of TeV gamma rays at energies between 10 and 100 TeV, and map the diffuse emission from the plane of our Galaxy. The HAWC (High Altitude Water Cherenkov) observatory builds on the experience and technology of Milagro to make a second-generation high-sensitivity detector. This unique detector will be capable of continuously surveying the TeV sky for steady and transient sources from 100 GeV to 100 TeV.

HAWC will be built by a collaboration of scientists from the US and Mexico with joint support. The HAWC site is Sierra Negra, Mexico, which is a very high altitude (4100m) site near existing infrastructure and collaborating universities. The HAWC observatory will utilize water Cherenkov technology (as proven by Milagro) and many of the Milagro components. The first phase of HAWC can be operational quickly, surpassing Milagro’s sensitivity within two years of the onset of funding. Because of the increased altitude, the increased physical area, and optimized design, HAWC will have an improved angular resolution, larger effective area, lower energy threshold and better background rejection. These improvements will result in a sensitivity of 10-15x (depending on source spectrum) better than that of Milagro and can be accomplished without any new technology, but only a modest upgrade to the existing electronics. We have used the existing Milagro data and simulations to verify these calculations.

HAWC will enable very high energy gamma-ray studies that are unattainable with the current suite of instruments:

1. HAWC will map the Galactic diffuse gamma-ray emission above 1 TeV and thereby measure the cosmic-ray flux and spectrum throughout the Galaxy. This map will allow us to look for regions of strong emission above that expected from correlations with matter: a signature of cosmic-ray acceleration.
2. HAWC with its improved angular and energy resolution plus enhanced background rejection will discover the highest energy gamma-ray sources in the Galaxy. Milagro has already observed gamma rays from one source, MGROJ1908+06, above 100 TeV. HAWC’s measurement of high-energy spectra will allow us to determine whether these sources are also sources of the galactic cosmic rays.
3. HAWC will perform an unbiased sky survey with a detection threshold of ~30 mCrab in two years, enabling the monitoring of known sources and the discovery of new classes of diffuse and point-like TeV gamma ray sources. HAWC, in one year, will be more sensitive at energies above ~6 TeV in its entire field of view than IACTs with 50 hours of observation on a point source.
4. With the sensitivity to detect a flux of 5 times that of the Crab in just 10 minutes over the entire overhead sky, HAWC will observe AGN flares that are unobservable by other instruments, including TeV orphan flares. Multi-wavelength observations of AGN flares from radio to TeV probe the environment up to within a few hundred AU of the super-massive black hole constraining models of gamma ray production and acceleration of charged particles.
5. HAWC’s low energy sensitivity and continuous operation are unique and essential to measure the prompt emission from gamma-ray bursts. HAWC can detect GRBs out to z~1 if, as predicted, their TeV fluence is comparable to their keV fluence, while for closer GRBs much lower fluences can be detected. If GLAST sees a single GRB photon above 100 GeV, HAWC will see hundreds, revealing the high energy behavior of GRBs and allowing us to probe the bulk Lorentz factor and size of the emitting region.

The Detector

HAWC will re-use the 900 8" Hamamatsu PMTs from Milagro, and deploy each PMT in a 4.6 m deep by 5.0 m diameter commercial plastic water tank. The tanks will be deployed in a dense pattern that provides more than 75% coverage of the 150m x 150m instrumented area. Each tank will contain an 8" baffled upward-facing PMT anchored to the bottom. When a high-energy gamma ray is incident on the atmosphere of the Earth, an electromagnetic Extensive Air Shower (EAS) is induced. If the energy is high enough, the secondary particles can arrive at the HAWC detector in a flat plane. The array of water tanks are sensitive to these secondary particles as they reach the ground. These energetic secondaries illuminate the PMT with the Cherenkov light they produce. When several tanks observe the same EAS, it is possible to reconstruct the direction of the primary gamma-ray that caused the air shower.

In addition to the gamma rays which strike the atmosphere, HAWC must contend with a large background of hadron-induced cosmic-ray EAS events. These hadronic background events differ from the gamma-ray events and allow us to distinguish between the two types of events. Qualitatively speaking, hadronic air showers are 'clumpy', and can have localized deposition of energy far from the shower core. Gamma-ray events are typically smooth. The dimensions of the HAWC tanks have been chosen to maximize our ability to see these local depositions, while maintaining the ability to see gamma-ray events.

HAWC's background rejection at the highest energies (> 50 TeV) is more than an order of magnitude better than Milagro's and will allow a nearly background-free measurement. This background rejection, combined with HAWC’s vastly superior energy resolution and angular resolution, will allow us to make a precision measurement of the highest energy gamma rays ever seen.

Last modified: October 04, 2007 09:50:08.