Clemson astrophysicists contribute to article in Science about a tiny but striking discovery
CLEMSON, South Carolina – Unlike an asteroid, when a particle far smaller than an atom strikes Earth, there is no explosion. Except in scientific circles.
For the first time, a team of international scientists has located the source of a high-energy subatomic particle that pierced our planet last year. After being hurled our way with stupendous force from a power-packed galaxy trillions of miles from the Milky Way, it landed in Antarctica.
The particle, called a neutrino, traveled 3.7 billion light years (each light year is equivalent to about 5.88 trillion miles) at almost the speed of light before it passed through the crust of Antarctica on Sept. 22, 2017, and interacted with a nucleus in the ice. The neutrino – the smallest known particle that contains mass – was produced by protons accelerated in a jet that was fueled by a supermassive black hole.
The results of the discovery were published in the July 12 volume of Science in two different articles: “Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A” and “Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert.” Clemson University astrophysicist Marco Ajello and postdoctoral scholar Vaidehi Paliya were among the co-authors of the first article.
A blazar galaxy contains a supermassive black hole with a mass that is millions to billions of times larger than our sun. A blazar propels jets of particles out of the galaxy in opposite directions. When one of these jets points directly toward Earth, they are especially bright.
“Victor Hess discovered in 1912 that our Earth is bombarded by cosmic rays containing charged particles, mostly protons, that are accelerated to energies much larger than those attainable in laboratories,” said Ajello, an assistant professor in the department of physics and astronomy. “But which kind of cosmic source could accelerate particles with such enormous power has remained a mystery – until now. For the first time, a neutrino has been detected that points back to a distant blazar.”
The discovery was made possible by the Fermi and IceCube collaborations, which bring together hundreds of scientists around the world, including Ajello and Paliya. The Fermi Large Area Telescope (Fermi-LAT) is housed on a NASA spacecraft that was launched in June 2008 to record images of high-energy gamma rays (the most powerful and energetic of all forms of light). The National Science Foundation funded IceCube Neutrino Observatory, an international operation run by 300 scientists from 12 nations that is located at the Amundsen-Scott South Pole Station. The observatory has more than 5,000 sensors distributed over a cubic kilometer of ice.
When an array of IceCube sensors detected the light produced by the neutrino-nucleus interaction in the ice, the chase was on to discover its origin. The IceCube team found the location in the sky by tracing the neutrino’s path back toward the constellation of Orion. The Fermi-LAT team alerted IceCube and the entire community that a blazar was in a high-activity state from the same position the neutrino had traveled during its fantastically long journey to Earth.
“Vaidehi and I contributed to this discovery as part of the Fermi-LAT team,” said Ajello, who helped develop the automated systems used to detect blazar flares. “This latest discovery opens a new window of multi-messenger astrophysics, together with the gravitational waves detected in conjunction with gamma-ray bursts (GRBs).”
By studying neutrinos, cosmic rays and gamma rays, scientists are able to form a better understanding of what occurs in turbulent environments such as stars, black holes and supernovas. Though tiny almost beyond comprehension, neutrinos are among the most plentiful particles in the universe, far outnumbering protons and electrons. The neutrino detected this past September struck the Antarctic ice with the energy of about 300 trillion electron volts – more than 45 times the energy achievable in the most powerful particle accelerator on Earth.
“The association of the IceCube-detected neutrinos with the gamma-ray outburst from a blazar has again proved the importance of the all-sky scanning operation of the Fermi-LAT,” Paliya said. “Since such blazar outbursts can happen anywhere in the sky, it’s essential to keep Fermi patrolling the sky as long as possible.”
– Information from a NASA media release was used in this report