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Neutrino emission by blazar
In this artistic rendering, a blazar is accelerating protons that produce pions, which produce neutrinos and gamma rays. One neutrino’s path is represented by a blue line passing through Antarctica, while a gamma ray’s path is shown in pink. (IceCube / NASA Illustration)

An array of detectors buried under a half-mile-wide stretch of Antarctic ice has traced the path of a single neutrino back to a supermassive black hole in a faraway galaxy, shedding light on a century-old cosmic ray mystery in the process.

The discovery, revealed today in a flurry of research papers published by the journal Science and The Astrophysical Journal, marks a milestone for the IceCube Neutrino Observatory at the National Science Foundation’s Amundsen-Scott South Pole Station.

It also marks a milestone for an observational frontier known as multi-messenger astrophysics, which takes advantage of multiple observatories looking at the sky in different ways. Thanks to IceCube’s alert, more than a dozen telescopes were able to triangulate on the neutrino’s source.

“No one telescope could have done this by themselves,” said IceCube lead scientist Francis Halzen, a physics professor at the University of Wisconsin at Madison.

The source of the high-energy neutrino detected last Sept. 22 appears to be a giant elliptical galaxy with a rapidly spinning black hole at its center, 3.7 billion light-years from Earth.

Such a galaxy is known as a blazar, and its signature feature is a pair of jets that spew radiation and subatomic particles along the axis of the black hole’s rotation. One of the blazar’s jets just happens to be pointed directly at Earth.

Astronomers have known about the blazar, known as TXS 0506+056 or the “Texas Source,” for years. But before IceCube came on the scene, they had no way of associating it with cosmic rays.

Cosmic rays are high-energy particles that reach Earth from space, and most of them carry an electrical charge. Such particles can be deflected by magnetic fields, or blocked by interactions with intervening matter. That makes it impossible to trace the particles’ paths to their source.

Neutrinos are different: They don’t carry an electrical charge, have virtually no mass, and interact so weakly with other types of matter that they typically pass right through anything that gets in their way — including stars and planets. That means they travel in a straight line from their source.

On rare occasions, a high-energy neutrino makes a direct hit on an atomic nucleus, setting off a subatomic chain reaction. It’s exactly that type of reaction that the $279 million IceCube Neutrino Observatory is designed to detect.

The heart of the observatory is a three-dimensional array with thousands of light sensors, spread across a cubic kilometer of crystal-clear Antarctic ice deep beneath the surface. When a neutrino hits a nucleus, it triggers a characteristic flash of blue light that points in the direction of the neutrino’s origin.

On Sept. 22, IceCube picked up on a strong flash and determined that it was sparked by a neutrino with an energy of about 300 trillion electron volts. That’s almost 50 times as energetic as the proton beams circulating in Europe’s Large Hadron Collider.

Within a minute, an alert went out to other astronomers to focus their telescopes on the patch of sky associated with the neutrino source in the constellation Orion.

The IceCube Neutrino Observatory is buried at depths between 1.5 and 2.5 kilometers below the South Pole. The only visible equipment is the IceCube Lab, which hosts the computers that collect data from over 5,000 light sensors in the ice. In this artwork, which incorporates a photo of the Ice Cube Lab, the signature of neutrino event IC170922 is shown in the ice. (IceCube Collaboration / NSF)

Over the days that followed, the Fermi Gamma-Ray Space Telescope and the MAGIC Telescope in the Canary Islands detected a strong gamma-ray burst coming from TXS 0506+056.

Other instruments, including the Neil Gehrels Swift Observatory and the NuSTAR X-ray telescope, picked up strong signals in multiple wavelengths from the same flaring source. And when IceCube’s scientists looked back through their archives, they found evidence of yet another flare that emanated from TXS 0506+056 in December 2014.

“All the pieces fit together,” Albrecht Karle, a senior IceCube scientist from UW-Madison, said today in a news release. “The neutrino flare in our archival data became independent confirmation. Together with observations from the other observatories, it is compelling evidence for this blazar to be a source of extremely energetic neutrinos, and thus high-energy cosmic rays.”

For more than a century, astronomers have speculated that cosmic rays emanated from violent phenomena such as supernovae, black holes and colliding galaxies. Now they have more than speculation to go on.

“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,” Halzen said. “Now, we have identified at least one source that produces high-energy cosmic rays because it produces cosmic neutrinos.”

Star chart
This star chart shows the location of the neutrino source, TXS 0506+056, as a blue set of crosshairs in the constellation Orion. The blazar is too distant and faint to be seen with the naked eye. (IceCube / NASA)

Like last year’s first-ever detection of a neutron star collision, IceCube’s findings demonstrate the power of multi-messenger astronomy. “The era of multi-messenger astrophysics is here,” NSF Director France Cordova said in a statement. “Each messenger — from electromagnetic radiation, gravitational waves and now neutrinos — gives us a more complete understanding of the universe, and important new insights into the most powerful objects and events in the sky.”

Cordova said “such breakthroughs are only possible through a long-term commitment to fundamental research and investment in superb research facilities.”

During an NSF news conference, Cordova said IceCube would continue its quest to piece together the cosmic ray puzzle, which ranks among other mysteries such as the nature of dark matter and dark energy. “Going after those mysteries … is what science is all about,” she said.

Uppsala University physicist Olga Botner, a former spokesperson for the IceCube Collaboration, said one of the next steps would be to combine neutrino observations from IceCube with gravitational-wave readings from LIGO and other observatories to get an even better picture of the universe’s most violent phenomena.

“We believe that this is the discovery waiting for us just around the corner,” she said.

“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,” are freely available on Science’s website. Another paper, titled “A Multimessenger Picture of the Flaring Blazar TXS 0506+056: Implications for High-Energy Neutrino Emission and Cosmic Ray Acceleration,” has been published by The Astrophysical Journal.

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