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University of Wisconsin–Madison

Solving the Mystery of Cosmic Rays


MADISON – With the help of an icebound detector situated a mile beneath the South Pole, an international team of scientists has found the first evidence of a source of high-energy cosmic neutrinos, ghostly subatomic particles that can travel in a straight line for billions of light-years, passing unhindered through galaxies, stars and anything else nature throws in its path.

The observation, made by the IceCube Neutrino Observatory at the Amundsen-Scott South Pole Station, helps resolve a more than century-old riddle about what sends subatomic particles such as neutrinos and high-energy cosmic rays speeding through the universe.

Since they were first detected more than a hundred years ago, cosmic rays – highly energetic particles that continuously rain down on Earth from space – have posed an enduring mystery: What creates and propels the particles across vast distances? Where do they come from?

Two papers published this week (July 13, 2018) in the journal Science include the first tangible evidence that a blazar – a giant elliptical galaxy with a massive, rapidly spinning black hole at its core – is the source of a high-energy neutrino detected Sept. 22, 2017 by the National Science Foundation-supported IceCube Observatory. A signature feature of blazars is twin jets of light and elementary particles that shoot like laser beams from the poles on the axis of the black hole’s rotation. (Although not visible to the naked eye, the galaxy, denoted by astronomers as TXS 0506+056, is situated in the night sky just off the left shoulder of the constellation Orion and is an estimated 4 billion light-years from Earth.)

Equipped with a nearly real-time alert system triggered when neutrinos of the highest energies crash into an atomic nucleus in or near the IceCube detector, the observatory – in less than a minute after the initial detection – relayed coordinates to telescopes worldwide for follow-up observations. Two gamma-ray observatories, NASA’s orbiting Fermi Gamma-ray Space Telescope and the Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC) in the Canary Islands, detected a flare of high-energy gamma rays associated with TXS 0506+056, a convergence of observations implicating the blazar as the most likely source.

The Fermi telescope first detected enhanced gamma-ray activity within less than 0.06 degrees of the neutrino pointer from IceCube, linking the high-energy photons that compose gamma rays to TXS 0506+056. It was the strongest gamma-ray flare in a decade of Fermi observations of the source, a known but little-studied galaxy. Follow-up observations by MAGIC detected gamma rays of even higher energies.

The gamma-ray observations show that the blazar is among the most luminous objects in the known universe and adds to the body of multi-messenger evidence that the blazar is powerful enough to accelerate high-energy cosmic rays and associated neutrinos.

Austrian physicist Victor Hess proved in 1912 that the charged particles scientists were detecting in the atmosphere were coming from space and not from other suspected sources on Earth such as radioactive elements. In the decades since, scientists speculated that the most violent objects in the cosmos, things like supernova remnants, colliding galaxies and the energetic black hole cores of galaxies known as active galactic nuclei were the potential sources of the particles. Curiously, blazars, a type of active galactic nucleus, were lower on the list. There are several thousand known high-energy blazars, some of the brightest objects in the sky.

“It is interesting that there was a general consensus in the astrophysics community that blazars were unlikely to be sources of cosmic rays, and here we are,” says Francis Halzen, a University of Wisconsin-Madison professor of physics and the lead scientist for the IceCube Neutrino Observatory. “Now, we have identified at least one source that produces high-energy cosmic rays because it produces cosmic neutrinos. Neutrinos are the decay products of pions. In order to produce them you need a proton accelerator.”

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