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2015 Featured Story #2: MSU Plays Major Role in Unique Gamma-Ray Observatory

MSU Participation in HAWC Oservatory

Members of the Michigan State University Department of Physics and Astronomy are major players in the new High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory in Puebla, Mexico, which will observe very high-energy gamma rays and cosmic rays coming from extreme sources in the universe, such as black holes, dark matter and exploding stars. HAWC is a collaborative effort between the United States and Mexico. The site was chosen for its high altitude (13,500 feet) and proximity to the LMT Alfonso Serano, a large radio telescope.

An inaugural event on March 19th & 20th marked the completion of the construction phase and the official start of HAWC’s five-year operational phase.

MSU is among the fourteen U.S. and ten Mexican universities in the collaboration, which brings together about 150 individuals. MSU Physics and Astronomy faculty members involved with the project include Jim Linnemann, professor and HAWC electronics coordinator, and Kirsten Tollefson, associate professor and deputy manager of HAWC detector operations. Tyce DeYoung, associate professor and deputy spokesperson for the IceCube collaboration (a gigaton-scale neutrino detector at the South Pole), is involved part-time with the HAWC facility, and there is participation by MSU students and post-doctoral researchers as well.

HAWC is designed to observe cosmic rays and TeV gamma rays (photons with one trillion times the energy of sunlight) with an instantaneous aperture that covers more than 15 percent of the sky. With this large field of view, HAWC will see up to two-thirds of the sky every 24 hours. Each of HAWC’s detectors is a huge tank containing 50,000 gallons of ultrapure water with four light sensors anchored to the floor. With 300 detectors spread over an area about the size of three football fields, HAWC is able to “see” these events in relatively high resolution.

HAWC’s detectors work by capturing Cherenkov radiation – flashes of visible light produced by charged particles hitting the HAWC detectors while moving faster than light can travel in the water. The speed of light in a vacuum is constant and as fast as anything can travel, but light slows down while traversing transparent materials such as glass or water (which is how optical lenses work). High energy particles can thus go faster than light can within the material, which generates the Cherenkov light as a side-effect. Millions of these particles are produced when a gamma ray strikes the atmosphere, but most are absorbed before reaching the ground. HAWC was built at high altitude to capture as many as possible.

HAWC’s predecessor, an observatory known as Milagro that operated near Los Alamos, New Mexico, ceased taking data in 2008. The completed HAWC Observatory will be 10 times more sensitive than Milagro.

MSU has taken a leadership role in HAWC since the project was first proposed. One of the first meetings was held at MSU in 2004. In 2010, MSU was involved with the original proposal put forward to the U.S. Department of Energy (DOE) and the National Science Foundation (NSF) to secure funding to build the facility. MSU was also responsible for the custom-built precision electronics that manage the timing for the experiment.

MSU postdoctoral fellow Tolga Yapici is in charge of the remote monitoring software. Also contributing to the project were graduate student Sam Marinelli, and Honors College undergraduates Max Paluska and Joshua Schroeder.

“HAWC truly is the only observatory of its kind, and will give us a clearer picture than ever before of the high-energy wonders of the universe,” Jordan Goodman, professor of physics at the University of Maryland has stated. Goodman is the U.S. lead investigator and spokesperson for the HAWC collaboration.

MSU Professor Jim Linnemann describes the HAWC Observatory's design features: “We watch the sky roll by every day and we inherently look at large swatches of sky all at the same time. This new survey instrument will enable us to get an unbiased view of the sky in very, very high-energy light. It allows us to look at unusual and exotic sources – like gamma ray bursts, which we might see only twice a year. Only rare and extreme astrophysical sources can produce such high-energy light: pulsating neutron stars, other remnants of supernova explosions, or jets of particles ejected from massive black holes at the center of other galaxies. HAWC is sensitive enough that it is expected to discover many more such sources.”

Discussing the scientific questions behind this research project, MSU Associate Professor Kirsten Tollefson said, “In our field of astroparticle physics, we’re answering the really big questions about how the universe was formed, and what the future of the universe is. Is our ultimate fate going to be a big crunch, or the big freeze? Will we expand forever, or collapse back in on ourselves?””

The MSU Faculty members also reported on future plans, now that the HAWC Observatory has been launched officially. “During the first two years of the project, we will be doing a fair bit of development of the analysis software, to get the most out of the apparatus,” Linnemann said. “We have already begun to talk about ideas for upgrades, even considering whether we can establish another facility in the southern hemisphere,” Tollefson added.

NSF, DOE and the Los Alamos National Laboratory provided funding for the United States’ participation in the HAWC project. El Consejo Nacional de Ciencia y Tecnología (CONACYT) is the primary funder for Mexican participation.

For additional information, see this MSU Today article.