Solving a Cosmic Mystery Daniel Wik, assistant professor of physics and astronomy, helped conclude a study using data from NASA’s NuSTAR space telescope to confirm that Eta Carinae, the most luminous and massive stellar system within 10,000 lightyears, is accelerating particles to ultra-high energies. Some of the particles could reach Earth as cosmic rays. John Belz
Gamma Rays and the Origins of Lightning John Belz, associate professor of physics & astronomy, first became interested in cosmic rays in the late 1990s. “There was an interesting, unsolved problem at the time,” said Belz. “Cosmic rays were observed with energies greater than predicted— something we hadn’t expected to see. Eventually the problem was resolved by Utah’s High Resolution Fly’s Eye detector.” The High Resolution Fly’s Eye or “HiRes” detector was an ultra-high-energy cosmic ray observatory located in the west desert of Utah from 1997-2006.
TA near Delta, Utah Today the Telescope Array Surface Detector (TA), a 700-square-kilometer observatory in Utah’s west desert, has replaced HiRes and detects high-energy particles that constantly collide with the Earth’s atmosphere from space. Belz serves as principal investigator of the National Science Foundationfunded Telescope Array Lightning Project, which uses data from the TA as well as a set of lightning detection instruments. His research focuses on lightning and gamma rays—the highest energy light waves on the electromagnetic spectrum—and he and his colleagues are trying to understand the mechanism by which the flash in lightning is initiated. Until 1994, satellites in the Earth’s orbit were expected to detect gamma radiation from major celestial events
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such as exploding stars. Then it was discovered that lightning could produce bursts of gamma rays, known as Terrestrial Gamma Flashes (TGFs), lasting about a millisecond and directed up towards space. In 2013, TA physicists detected the downward-directed counterpart of TGFs in which the radiation is beamed towards the Earth’s surface. This discovery has provided new information and additional questions about how, where, and when the lightning flash begins.
New Detectors “As common as atmospheric lightning is,” said Belz, “we don’t completely understand how it works. The practical application of our research is that it may help us better understand how gamma rays are produced and the nature of lightning and the lightning initiation process.” This past summer the team deployed new detectors at the TA site, including a radio interferometer, that will help them see in greater detail how the gamma rays are produced at the beginning of the flash. After obtaining a Ph.D. from Temple University, Belz did postdoctoral work at Rutgers University and held faculty positions at Montana State and the University of Montana. He joined the U’s Physics & Astronomy department in 2005 as a visiting assistant professor and became an associate professor in 2014.
“The key to accurately measuring Eta Carinae’s X-rays and identifying the star system as the gamma ray source—and thus proving that the colliding winds of this binary system are accelerating cosmic rays—was to fully characterize NuSTAR’s background,” said Wik. Wik previously developed a multi-component background model for the NuSTAR mission, but Eta Carinae’s location in the plane of the Milky Way caused the background of NuSTAR’s nine separate observations to be more complicated than usual. He helped identify additional sources of background and how to account for them, allowing the link between Eta Carinae’s X-ray and gamma ray emissions to become clear. Eta Carinae, located about 7,500 light-years away, contains a pair of massive stars whose eccentric orbits bring them unusually close to Earth every 5.5 years. The two stars contain 90- and 30-times the mass of our Sun, and they pass 140 million miles apart at their closest approach—about the average distance between Mars and the Sun. Adapted from an original release written by Francis Reddy of NASA’s Goddard Space Flight Center, available here: https://go.nasa.gov/2knv825.
Daniel Wik
Eta Carinae’s great eruption in the 1840s created the billowing Homunculus Nebula, imaged here by Hubble. Now about a light-year long, the expanding cloud contains enough material to make at least 10 copies of our Sun. Astronomers cannot yet explain what caused this eruption. Photo credit: NASA, ESA, and the Hubble SM4 ERO Team.
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