The High Altitude Water Cherenkov (HAWC) Observatory

Dr. Wayne Springer
Department of Physics and Astronomy
University of Utah

The High Altitude Water Cherenkov (HAWC) observatory is a continuously operated, wide field of view detector principally designed to observe astrophysical sources of gamma rays. Copious numbers of secondary particles from the extensive air showers (EAS) produced by the collision of gamma and cosmic rays with nuclei in the atmosphere reach HAWC at the altitude of 4100m on Sierra Negra Mountain in Mexico.  HAWC consists of an array of three hundred closely spaced water tanks covering an area of approximately 22,000m2. Each tank, 7.3m in diameter and 4.5 m deep, contains approximately 200,000 liters of water and is instrumented with four photomultiplier tubes that provide timing and charge information for the Cherenkov light produced by the EAS particles. The altitude and size of HAWC results in a detector with significant sensitivity to gamma rays and cosmic rays with energies from several hundred GeV up to several hundred TeV. HAWC can reliably estimate the energy and arrival direction of cosmic and gamma rays arriving from zenith angles of up to 450, resulting in an instantaneous field of view of approximately 2 steradians. As the Earth rotates over one day, HAWC observes a swath of the sky from -260 to 640 in Declination and 0 to 24 hours in Right Ascension, roughly 2/3 of the sky. HAWC has been optimized to study transient and steady emission from both galactic and extragalactic sources of gamma rays.  These capabilities enable HAWC to serve as a survey instrument for multi-wavelength studies of transient phenomenon with other instruments available in the gamma-ray community. The wide field of view of HAWC provides the ability to observe extended objects such as Pulsar Wind Nebulae and diffuse emission of gamma rays. HAWC has significant discovery potential, including the possibility of indirect detection of dark matter through the observation of gamma rays produced via dark-matter particle annihilation. Given the large number of cosmic-ray events seen by HAWC, measurements of the energy and arrival direction dependence of the cosmic ray flux are also performed. Our group at Utah is using HAWC to observe muons with nearly horizontal trajectories to study properties of cosmic rays. HAWC commenced data-taking operations with the completed detector on March 20, 2015.  A discussion of the detector design, science capabilities and results will be presented.