The High Altitude Water Cherenkov Observatory (HAWC), operating in central Mexico, is gathering information about high-energy gamma rays as they enter the Earth’s atmosphere. Photo:Ignacio Taboada.
In the shadow of Mexico’s Sierra Negra volcano, an array of 300 silvery water-filled tanks is capturing the calling cards left by powerful visitors from our galaxy and beyond.
The tanks are the most visible components of the High Altitude Water Cherenkov Observatory (HAWC), a one-of-its-kind facility designed to gather information about high-energy gamma rays entering the Earth’s atmosphere.
Information about these gamma rays, which shower the Earth with charged particles, could expand our knowledge of black holes, supernovae, and other cosmic gamma ray sources. Built through a partnership between the United States and Mexico, HAWC adds yet another component to the toolbox of techniques astrophysicists can use to study the universe.
“We are conducting a survey in space and in time,” said Ignacio Taboada, an associate professor in Georgia Tech’s School of Physics who was involved in the design and construction of HAWC and now serves as science analysis coordinator for the facility. “There are regions of the sky that nobody has examined in detail. We may find something entirely new, especially outside the galactic plane.”
Through HAWC, scientists will gather information on gamma rays that carry about a trillion times more energy than the photons that enter our eyes and allow us to see. When one of these gamma rays collides with Earth’s upper atmosphere, it produces a shower of particles that rain down through the lower atmosphere in a pancake pattern, at almost the speed of light.
At times, these particles may pass through HAWC’s water- filled tanks, creating a flash of blue light known as Cherenkov radiation. That light, sometimes just a handful of photons, will be captured by four photomultiplier tubes located at the base of each light-tight tank. By measuring the intensity of the light and comparing the nanosecond dierences in its arrival times at dierent tanks in the array, scientists will be able to compute the energy of the gamma rays and also the direction from which they came.—JOHN TOON