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 is capable of continuously surveying the TeV sky for steady and transient sources from 100 GeV to 100 TeV.

HAWC has been built by a collaboration of scientists from the US and Mexico with joint support. The project has been fully operational since March 2015. Click here to read about the inauguration. The HAWC site is Sierra Negra, Mexico, which is a very high altitude (4100m) site near existing infrastructure and collaborating universities. The HAWC observatory utilizes water Cherenkov technology (as proven by Milagro) and many of the Milagro components. Because of the increased altitude, the increased physical area, and optimized design, HAWC has an improved angular resolution, larger effective area, lower energy threshold and better background rejection. These improvements result in a sensitivity of 10-15x (depending on source spectrum) better than that of Milagro and have been accomplished with only a modest upgrade to the existing electronics. We have used the existing Milagro data and simulations to verify these calculations.

HAWC enables very high energy gamma-ray studies that were previously unattainable:

  1. HAWC is mapping the Galactic diffuse gamma-ray emission above 1 TeV and measuring 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 is 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 can 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.