On Sept. 22, 2017, a ghostly particle ejected from a far distant supermassive black hole zipped down from the sky and through the ice of Antarctica at just below the speed of light, with an energy of some 300 trillion electron volts, nearly 50 times the energy delivered by the Large Hadron Collider at CERN, the biggest particle accelerator on Earth.
IceCube Neutrino Observatory
The cosmic invader –a messenger from the depths of extragalactic space, carrying secrets from some of the most extreme physics in the universe– triggered a cacophony of code-red detectors in the IceCube Neutrino Observatory (image below) located at the Amundsen-Scott South Pole Station, perhaps solving one of the enduring mysteries of physics and the cosmos.
As recently as February, 2021, the observatory saw evidence of a neutrino from a star being ripped apart by a black hole.
The Observatory is built into a cubic kilometer of clear glacial ice at the South Pole, detecting neutrinos when they interact with atoms in the ice. This triggers a cascade of fast-moving charged particles that emit a faint glow, called Cherenkov light, as they travel, which is picked up by thousands of optical sensors strung throughout IceCube. Scientists determine the energy of an incoming neutrino by the amount of light its particle cascade emits.
Neutrinos are the fastest, lightest, most unsociable and least understood fundamental particles, and scientists are just now capable of detecting high-energy ones arriving from deep space. They have no electrical charge and so little mass that it has not been accurately measured. They interact with other matter only by gravity and the weak nuclear force and thus flow through Earth and even miles of lead like ghosts.
One of the first plausible association between a single extragalactic object and one of these cosmic neutrinos occurred On Dec. 4, 2012, when a neutrino from a dramatic explosion, in a distant galaxy known as PKS B1424-418, that happened 10 billion years ago began reaching Earth.
The detection, an event now known as Big Bird, was of a neutrino with an energy exceeding 2 quadrillion electron volts (PeV). To put that in perspective, it’s more than a million million times greater than the energy of a dental X-ray packed into a single particle thought to possess less than a millionth the mass of an electron. Big Bird was the highest-energy neutrino ever detected at the time.
Although neutrinos far outnumber all the atoms in the universe, they rarely interact with matter, which makes detecting them quite a challenge. But this same property lets neutrinos make a fast exit from places where light cannot easily escape — such as the core of a collapsing star — and zing across the universe almost completely unimpeded. Neutrinos can provide information about processes and environments that simply aren’t available through a study of light alone.
IceCube found first evidence for a flux of extraterrestrial neutrinos, which was named the Physics World breakthrough of the year in 2013. To date, the science team of IceCube Neutrino has announced about a hundred very high-energy neutrinos and nicknamed the most extreme events after characters on the children’s TV series “Sesame Street.”
Big Bird Event–”Like a crime-scene investigation”
On Dec. 4, 2012, IceCube detected an event now known as Big Bird, a neutrino with an energy exceeding 2 quadrillion electron volts (PeV) was detected. To put that in perspective, it’s more than a million million times greater than the energy of a dental X-ray packed into a single particle thought to possess less than a millionth the mass of an electron. Big Bird was the highest-energy neutrino ever detected at the time and still ranks second.
Where did Big Bird come from? The best IceCube position only narrowed the source to a patch of the southern sky about 32 degrees across, equivalent to the apparent size of 64 full moons.
“It’s like a crime scene investigation”, says lead author Matthias Kadler, a professor of astrophysics at the University of Würzburg in Germany, “The case involves an explosion, a suspect, and various pieces of circumstantial evidence.”
The international team of astronomers, led by Kadler, professor for astrophysics at the university of Würzburg, and including other scientists from the new research cluster for astronomy and astroparticle physics at the universities of Würzburg and Erlangen-Nürnberg, have shown that a record-breaking neutrino seen around the same time likely was born in the same event.
Starting in the summer of 2012, NASA’s Fermi satellite witnessed a dramatic brightening of PKS B1424-418, an active galaxy classified as a gamma-ray blazar. An active galaxy is an otherwise typical galaxy with a compact and unusually bright core. The excess luminosity of the central region is produced by matter falling toward a supermassive black hole weighing millions of times the mass of our sun. As it approaches the black hole, some of the material becomes channeled into particle jets moving outward in opposite directions at nearly the speed of light. In blazars one of these jets happens to point almost directly toward Earth.
The term blazar comes partly from BL Lacertae, a starlike object that turned out to be the first of these objects ever recognized.
Fermi LAT images above show the gamma-ray sky around the blazar PKS B1424-418. Brighter colors indicate greater numbers of gamma rays. The dashed arc marks part of the source region established by IceCube for the Big Bird neutrino (50-percent confidence level). Left: An average of LAT data centered on July 8, 2011, covering 300 days when the blazar was inactive. Right: An average of 300 active days centered on Feb. 27, 2013, when PKS B1424-418 was the brightest blazar in this part of the sky.
During the year-long outburst, PKS B1424-418 shone between 15 and 30 times brighter in gamma rays than its average before the eruption. The blazar is located within the Big Bird source region, but then so are many other active galaxies detected by Fermi.
The scientists searching for the neutrino source then turned to data from a long-term observing program named TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry). Since 2007, TANAMI has routinely monitored nearly 100 active galaxies in the southern sky, including many flaring sources detected by Fermi. Three radio observations between 2011 and 2013 cover the period of the Fermi outburst. They reveal that the core of the galaxy’s jet had been brightening by about four times. No other galaxy observed by TANAMI over the life of the program has exhibited such a dramatic change.
The TANAMI is a multiwavelength monitoring program of active galaxies in the Southern sky. It includes regular radio observations using the Australian Long Baseline Array (LBA) and associated telescopes in Chile, South Africa, New Zealand and Antarctica. When networked together, they operate as a single radio telescope more than 6,000 miles across and provide a unique high-resolution look into the jets of active galaxies.
The radio images below from the TANAMI project reveal the 2012-2013 eruption of PKS B1424-418 at a radio frequency of 8.4 GHz. The core of the blazar’s jet brightened by four times, producing the most dramatic blazar outburst TANAMI has observed to date.
“Within their jets, blazars, about 1 billion solar masses in size, are capable of accelerating protons to relativistic energies. Interactions of these protons with light in the central regions of the blazar can create pions. When these pions decay, both gamma rays and neutrinos are produced,” explains Karl Mannheim, a coauthor of the study and astronomy professor in Würzburg, Germany.
“We combed through the field where Big Bird must have originated looking for astrophysical objects capable of producing high-energy particles and light,” adds coauthor Felicia Krauß, at the University of Erlangen-Nürnberg in Germany. “There was a moment of wonder and awe when we realized that the most dramatic outburst we had ever seen in a blazar happened in just the right place at just the right time.”
The team suggests the PKS B1424-418 outburst and Big Bird are linked, calculating only a 5-percent probability the two events occurred separately and are not related to each other. Using data from Fermi, NASA’s Swift and WISE satellites, the LBA and other facilities, the researchers determined how the energy of the eruption was distributed across the electromagnetic spectrum and showed that it was sufficiently powerful to produce a neutrino at PeV energies.
“Taking into account all of the observations, the blazar seems to have had means, motive and opportunity to fire off the Big Bird neutrino, which makes it our prime suspect,” explains Matthias Kadler.
“We have found the first source of cosmic rays”
Francis Halzen, the principal investigator of IceCube at the University of Wisconsin-Madison, and not involved in this study, prophetically said that the Big Bird result is an exciting hint of things to come.
“IceCube is about to send out real-time alerts when it records a neutrino that can be localized to an area a little more than half a degree across, or slightly larger than the apparent size of a full moon,” he observed. “We’re slowly opening a neutrino window onto the cosmos.”
Fast forward to where we began at the top of the page on September 22, 2017 when the prophetic Halzen announced: “We have found the first source of cosmic rays.”
One of the brightest objects in the universe
Within seconds Ice Cube had alerted a global network of astronomical satellites, including the Fermi Gamma-ray Space Telescope that traced the neutrino back to the supermassive black hole, an ancient quasar in a distant galaxy known as TXS 0506+056, one of the brightest objects in the universe, some 4 billion light-years away, was having an extreme outburst of X-rays and gamma-rays.
The resident Ice Cube scientists scoured their previous data and found that there had been previous outbursts of neutrinos from the galaxy, which they nicknamed the “Texas source,” including an enormous neutrino outburst in 2014 and 2015, providing a long sought clue to one of the enduring questions: where does the rain of high-energy particles from space known as cosmic rays come from?
“Where exactly in the active galaxy, the neutrinos are produced will be a matter of debate,” Halzen added.. “It is clear that the supermassive black hole provides the accelerator power,” he said, but how is a mystery.
Beginning of high-energy neutrino astronomy
“I think this is the real thing,” said John Learned, a neutrino expert at the University of Hawaii who is not part of Ice Cube, in an email to Dennis Overbye at the New York Times, “the true beginning of high energy neutrino astronomy, of which we have dreamed for many decades.” Now, he added, “we will start seeing into the guts of the most energetic objects in the universe.”
The lure of neutrinos for astronomy is that it is possible to trace them back to their origins. Not only do they fly long distances and from otherwise impenetrable spots like the cores of stars at virtually the speed of light, but by not having an electrical charge they are not affected by interstellar and intergalactic magnetic fields and other influences that scramble the paths of other types of cosmic particles, like protons and electrons. Neutrinos go as straight through the universe as Einsteinian gravity will allow.
IceCube was built, Halzen said, to find the source of cosmic rays –which are not actually “rays” of radiation but rather subatomic particles, such as protons or even entire nuclei, zipping through space approaching that of light. The observatory has been recording neutrinos ever since it started working in 2011, but had not been able to pinpoint the sources of any of them until now. One reason, he said, was that the scientists had assumed the sources would be nearby, perhaps even in our own Milky Way galaxy.
The Daily Galaxy, Avi Shporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research via The New York Times, Scientific American and University of Wurzburg. Avi was formerly a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL).
Image credit: top of page Shutterstock License
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