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Scientists trace neutrino to source outside our galaxy

IceCube lab in Antarctica under the stars.
IceCube lab. Photo: Martin Wolf/IceCube/NSF

In a find that advances our understanding of fundamental particles in the universe, scientists announced Thursday they've detected a high-energy neutrino from outside our galaxy and, for the first-time, pinpointed its source.

Why it matters: The evidence, detailed in two new studies in the journal Science, further demonstrate the potential for multi-messenger astronomy — that is, astronomy that looks at the whole electromagnetic spectrum — to help scientists answer longstanding mysteries about high-energy physics.

Where it came from: Evidence from a slew of observatories suggests the neutrino likely originated in an active galaxy known as a blazar. The blazar — TXS 0506+056 — is located about 4 billion light-years from Earth.

  • Like the Milky Way, blazars contain a supermassive black hole, but unlike our own galaxy the black hole is actively dragging in stars and dust and spewing out matter and light in a jet that happens to point toward Earth.

The high-energy neutrino was detected using an array of 5,000 sensors drilled deep underneath the South Pole, known as the IceCube Neutrino Observatory.

What they are: Neutrinos are subatomic particles with no electrical charge and almost no mass. They rarely interact with matter: A few hundred billion neutrinos move through every square inch of everything — human, home and planet — every second.

"I call them the snobs of the universe because they don't interact very much."
— Roopesh Ojha, Fermi Gamma-ray Space Telescope at NASA's Goddard Space Flight Center and the University of Maryland Baltimore County

Because they don't interact much, neutrinos are difficult to detect. But on the plus side, Ojha says, it also means they aren't being deflected by magnetic fields (like cosmic rays) or diffused by clouds (like light) and can carry unadulterated information about where they came from.

The IceCube Observatory's detection, made on Sept. 22, 2017, was backed up by findings from an array of Earth and space-based assets that scan the electromagnetic spectrum from radio waves to high energy gamma rays.

"It is a new sense. It is an entirely new means for us to learn about the cosmos," says Ojha.

Background: Cosmic rays — protons and other nuclei with high energies that travel through the universe at nearly the speed of light — were first discovered in 1912. Lower energy neutrinos have been detected from the sun and from a supernova. But, if the new findings hold up, this would be the first high-energy one whose origin has been traced back to a source outside our galaxy.

"We don’t really understand how the universe can be accelerating particles to the energies its reaching. We can’t do it on Earth. There is a lot to ask about how these sources can be producing things along these lines."
— MIT particle physicist Janet Conrad and member of the IceCube team

How they did it: The IceCube Neutrino Observatory detects radiation created by neutrinos on the extremely rare occasion that they interact with a proton or neutron in an atom. It's the optical equivalent of a sonic boom, Ojha told Axios.

On September 22, 2017, the observatory detected a single neutrino coming from the blazar TXS 0506+056. Within a minute, the observatory automatically alerted nearly two dozen space and ground telescopes around the world that then began to search the region for gamma rays, X-rays and visible light that are also expected to be produced at the same time as neutrinos.

The researchers then went back through 9.5 years of IceCube data and found that between September 2014 and March 2015, there were "an excess of high-energy neutrino events." Conrad says, "That was the thing that nailed it for me. Maybe we just got lucky but to me it makes it unlikely it is an accident."

Yes but: It is just one neutrino from a single blazar, and it's not a blazar that had caught scientist's attention before, says Neil Weiner, a physics professor at NYU's Center for Cosmology and Particle Physics, via email. Weiner was not involved in the new studies.

"This result gives evidence that these objects may explain some of these mysterious neutrinos. Of course, this is one object, and, convincing though the case is, you’d love to see another," Weiner said.

And, the researchers are careful to point out that the statistical significance they report puts the detection in the realm of "evidence" rather than a "discovery." Erik Blaufuss, a physicist at the University of Maryland and a member of the IceCube team, says, "We’re definitely claiming this is at the evidence level." 

The big picture: The new studies demonstrate the maturing of multi-messenger astronomy, in which scientists see and hear the universe using a multitude of observatories in order to learn more about a phenomenon.

"“We’re opening whole new horizons in which we’re viewing the universe,” said Julie McEnery, the project scientist for the Fermi gamma-ray Space Telescope and an astrophysicist with NASA.

Editor's note: The headline was clarified to show the source of a neutrino from outside the Milky Way was found, not the first detection of a neutrino from outside the galaxy.

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