The HAWC Gamma-Ray Observatory begins operations at the Sierra Negra volcano in Puebla, Mexico

A photograph of the HAWC observatory taken in August 2013 from the summit of the Sierra Negra. The image has been altered to show HAWC as it will appear when construction is complete in 2014. The 111 Cherenkov detectors currently installed (100 of which are in operation) are colored white and located in the upper right quadrant of the array.

On August 1, 2013, the High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory formally began operations. HAWC uses a unique detection technique to study the origin of very high-energy cosmic rays and observe the most energetic objects in the known universe. This extraordinary observatory differs from the classical astronomical design of mirrors, lenses, and antennas, instead observing particle interactions.

“HAWC can see the entire northern sky, all day,” explains Professor Stefan Westerhoff of the University of Wisconsin–Madison Department of Physics and the Wisconsin IceCube Particle Astrophysics Center. “Other similar Cherenkov telescopes are pointed in a specific area of the sky and only operate at night. HAWC can see phenomena in the Universe that flare for a short amount of time, and the hope is that we can see how high in energy gamma-ray bursts actually go.”

UW–Madison faculty, research staff, and students have contributed to the construction and operations of the observatory, providing expertise in the data acquisition system and electronics testing. Each week there are two people who travel up the Sierra Negra mountain site daily to operate the detector and assist in the ongoing construction.

“In a lot of projects, the students never see the experiment and it’s abstract for them. Traveling to the site enables them to see and understand the detector,” says Westerhoff, who leads the HAWC research group at UW–Madison, one of thirty-one institutions from the US and Mexico that form the HAWC Collaboration.

Physics graduate student Ian Wisher has visited the site over a half dozen times, working on electrical wiring, helping build support structures for buildings, and configuring a calibration system. “It’s nice going there and working on site, getting physical interactions with the detector,” he says.

The observatory, which is still under construction and about one-third completed, uses an array of detectors to observe cosmic rays and gamma rays at energies between 100 GeV and 100 TeV. HAWC is located at an altitude of 4100 meters on the slope of the Sierra Negra and Pico de Orizaba volcanoes, giving it an ideal location to observe particle showers and cosmic radiation.

HAWC is optimized to detect gamma rays, high-frequency electromagnetic radiation, and cosmic rays, high-energy subatomic particles. Both are products of the most energetic and cataclysmic events in the known universe: phenomena such as the collision of two neutron stars, supernova explosions, and active galactic nuclei, which host black holes millions of times more massive than the Sun.

Both cosmic rays and gamma rays reach the detector as particle showers. Gamma-ray radiation interacts with air molecules in the upper atmosphere, creating a shower of particles that reaches the surface of Earth in a “cascade” of particles.

Currently, 110 out of 300 detector tanks are deployed and taking data. Each tank is 5 meters high and 7.3 meters in diameter, storing 180,000 liters of extrapure water and four highly sensitive light sensors. When particles originating from a gamma-ray shower or from cosmic rays travel through the tank, the sensors detect a burst of radiation known as Cherenkov light. That light enables physicists to reconstruct the direction and intensity of the particle, making astrophysics possible and providing a new view of the Universe.

Working in the HAWC Collaboration provides students and postdocs with a new view of the world as well. “It’s a great experience working with people from a different culture. It’s just going to become more common. There are some challenges, balancing the requirements of two countries and cultures,” says Wisher, “but overall I really enjoy it. There is always stuff to do, and it’s empowering to see the detector being built and know I contributed to it.”

The construction and operation of HAWC has been made possible by the financial support of several Mexican institutions including the Consejo Nacional de Ciencia y Tecnología (CONACYT), the Universidad Nacional Autónoma de México (UNAM), and the Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE). Funding has also been provided by the United States through the National Science Foundation (NSF), the Department of Energy (DOE), Los Alamos National Laboratory (LANL), and the University of Maryland.

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