Out there in space is a class of objects called blazars. Think of them as extreme particle accelerators, capable of gathering energies a million times stronger than the Large Hadron Collider in Switzerland. They turn out to be the culprits of one of the great mysteries of astrophysics: what creates and propels neutrinos through the universe at lightning speeds? It turns out the answer has been there all along: blazars pump out neutrinos and cosmic rays. That’s the conclusion reached by a group of astronomers led by Dr Sara Buson of the Universität Wurzburg in Germany as they studied data from a very unique facility here on Earth: the IceCube observatory. Neutrino in Antarctica.
Understanding the Origins of Speed Demon Particles
Neutrinos are funny little ducks in the astrophysical zoo. They come from cosmic ray interactions in blazars and have very little mass. Neutrinos do not interact with matter as they travel through the cosmos, which means they travel through galaxies and planets. They pierce you even as you sit here and read this, and leave very little evidence of their passage. Fortunately, this last characteristic means that they can be traced back to their sources since electromagnetic forces don’t even bother them.
So how did Buson and his team find the places where neutrinos are born? They turned to IceCube, which is buried deep in the ice at the South Pole. It is the most sensitive neutrino detector on the planet. It searches for these nearly massless subatomic particles, which astronomers also like to call astrophysical messengers. This is because they contain information about violent astrophysical events and sources – like black holes, neutron stars – and blazars.
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In 2017, IceCube detected a neutrino from blazar TXS 0506+056. It is the active nucleus of a distant galaxy which is brighter than its entire galaxy. The data carried by the neutrino told the team that it came from the core of this blazar and had traveled 5.7 billion light-years to be measured by IceCube. It doesn’t just send out neutrinos, it’s also a light radio source that pumps light through the electromagnetic spectrum. (For the astronomers among us, this blazar is in the direction of the left shoulder of the constellation Orion.)
Of course, TXS 0506+056 is not the only source of neutrinos (apart from the Sun for example). IceCube found 19 “hot spots” in the southern sky. At least ten of them are most likely blazars. “The results provide, for the first time, compelling observational evidence that the subsample of PeVatron blazars are extragalactic neutrino sources and thus cosmic ray accelerators,” Buson said in a press release.
PeVatron blazars accelerate particles to at least PeV energies. PeV is the abbreviation of “peta electron volt” and is equal to 1015 electron-volts. To give you an idea of its power, the Large Hadron Collider reached just over 1 PeV in 2015.
Neutrinos and multi-messenger astronomy
These nearly massless, high-speed cosmic rays and neutrinos are the last “messengers” from the distant universe. For a long time, astronomers have used light to study the universe. But it’s not the only messenger that can tell us about stars, planets, galaxies, black holes and other objects in the cosmic zoo. Neutrinos, cosmic rays, and gravitational waves provide other means of messaging carrying valuable information about distant astrophysical events and objects.
According to Clemson University team member Marco Ajello, multi-messenger astronomy adds significantly to our understanding of the universe. “It’s like feeling, hearing and seeing at the same time. You will have a much better understanding,” he said. “The same is true in astrophysics, because the information you get from multiple detections of different messengers is much more detailed than what you can get from light alone.”
The data provided by neutrinos and other messengers from the distant universe opens the way to a better understanding of objects like the blazars that create them. Team members will now focus on why and how blazars accelerate particles like neutrinos. Obviously, they are extremely energetic objects in their own right. Blazar TXS 0506+056 is a typical active galactic nucleus powered by a supermassive black hole. It has a relativistic jet pointing directly at us here on Earth, but luckily we’re too far away to be hurt by it. Instead, we can observe how it generates neutrinos. It is, in fact, the first known source of astrophysical neutrinos – and one of the first providers of multi-messenger astronomy. Now astrophysicists have a whole new set of objects that act as probes into the distant universe.
For more information
Astrophysicists prove that neutrinos come from blazars
Beginning a journey through the universe: the discovery of the extragalactic neutrino