In the shadow of Mexico’s tallest mountain, an array of 300 water-filled silver 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. The facility officially began operation March 20.
“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 the data 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 rains 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 differences in its arrival times at different tanks in the array, scientists will be able to compute the energy of the gamma rays and also the direction from which they came.
“We can do a new kind of astronomy with very high energy gamma rays,” said Taboada, who supervised design and construction of the system that monitors the health of HAWC’s 1,200 sensitive photomultiplier tubes.
HAWC works as a survey instrument, capturing information from nearly two-thirds of the sky with each rotation of the Earth. Because it can detect Cherenkov radiation day and night, HAWC could capture the first signs of a new gamma ray source, alerting astronomers to use more narrowly-focused instruments to gather more detailed data.
“HAWC will be looking all the time in all directions, and that is advantageous for catching things that change quickly in time or discovering things in the sky that nobody has looked at before,” Taboada added.
Taboada is specifically interested in transient events that take place in a thousand seconds or less. Once events of interest are identified, the facility will allow long-term study of objects that are typically observed only when they are bright enough to command attention.
“Supermassive black holes often produce jets that are variable over time,” Taboada explained. “Very few of these have been studied in an unbiased way. We need to be able to look at a source like that all the time, and HAWC will allow us to do that.”
Like many modern observatories, HAWC will produce massive amounts of data – upwards of 10 terabytes per day. Rough analysis on-site will reduce the amount of data, but finding the interesting needle in this haystack will require advanced data analysis. Complicating that task is filtering out cosmic rays, which are detected more frequently than the gamma rays.
HAWC was built at a location 13,500 feet above sea level on the slopes of Volcán Sierra Negra in central Mexico. The location was chosen because its high altitude shortens the distance the particles must travel to be detected.
Each of the detector tanks, which are 4 ½ meters high and 7 ½ meters in diameter, holds 50,000 gallons of ultra-pure water. The 300 tanks are spread across an area the size of three football fields. Construction required nearly six years and cost approximately $14 million, supported by the National Science Foundation, Department of Energy and the Consejo Nacional de Ciencia y Tecnologia (CONACYT) in Mexico.
The capabilities of HAWC complement those of the IceCube Neutrino Observatory located at the South Pole. Taboada is involved in that observatory, too, and says that while the two facilities measure different types of cosmic visitors, they can tell a more complete story together.
“There are sources that produce neutrinos that are also expected to produce gamma rays, so there are objects we should observe jointly between HAWC and IceCube,” he said. “The information you get from looking at neutrinos and gamma rays are different, so you can look at the same object in the sky in different ways, allowing us to learn more.”
Taboada cautions that HAWC conducts basic science that helps us learn more about the universe – without specific applications in sight.
“Basic science never answers the question of what this is useful for,” he said. “But if you look at the history of modern science, you will find over and over again that basic science has resulted in dramatic benefits to humankind.”
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Writer: John Toon