Last summer, scientists found a “ghost particle” called a neutrino in Antarctica. It traveled 3.8 billion light-years from a distant galaxy to reach Earth.
Because astronomers were able to trace the path of the neutrino back to the galaxy, known as TXS 0506+056, they wanted to know more about the galaxy. The galaxy sits to the left of Orion’s shoulder in his constellation. What made it unique and why did a neutrino come from this galaxy?
Researchers published their findings to answer these questions Wednesday in the journal Astronomy and Astrophysics.
“It was a bit of a mystery, however, why only TXS 0506+056 has been identified as neutrino emitter,” said Silke Britzen, a co-author of the study at the Max Planck Institute for Radio Astronomy. “We wanted to unravel what makes TXS 0506+056 special, to understand the neutrino creation process and to localize the emission site and studied a series of high-resolution radio images of the jet.”
Researchers studied observations of the galaxy taken between 2009 and 2018 to understand the galaxy’s activity before and after the neutrino event, known as IceCube 170922A.
Now, the researchers believe the cause of the neutrino was a collision within the galaxy of accelerated material, called a jet, close to a supermassive black hole. They think this led to more neutrino activity during an earlier flare between September 2014 and March 2015.
Neutrinos are referred to as ghostly because they are extremely vaporous particles that can pass through any kind of matter without changing. They have almost no mass. They can travel through the most extreme environments, like stars, planets and entire galaxies, and remain the same. Before July 2018, only two sources had been found: the sun and a supernova.
When the initial studies of the neutrino were released last year, astronomers said the galaxy where the neutrino originated had a supermassive, rapidly spinning black hole at its center, known as a blazar.
Now, the researchers think there are possibly two supermassive black holes in the central region of this galaxy. The black holes act as an engine powering the galaxy, making it one of the most energetic objects in the universe.
A blazar is a giant elliptical galaxy with two jets emitting light and particles moving close to the speed of light along the axis of the black hole’s rotation. They can flare for minutes or months. One of those blazingly bright jets — hence the name blazar — is pointing at Earth.
But there was an unexpected interaction of the jet material, where new material crashed into older jet material. It’s possible that the jets were curved, allowing this cosmic collision to happen. Either way, it leads to a neutrino.
“This collision of jetted material is currently the only viable mechanism which can explain the neutrino detection from this source,” said Markus Böttcher, a physics professor at South Africa’s North-West University who co-authored the study with Britzen. “It also provides us with important insight into the jet material and solves a long-standing question whether jets are leptonic, consisting of electrons and positrons, or hadronic, consisting of electrons and protons, or a combination of both. At least part of the jet material has to be hadronic — otherwise, we would not have detected the neutrino.”
Scientists have thought for some time that energetic particles could be created by these jets because they could act as “cosmic accelerators,” affecting protons and neutrons and turning them into cosmic rays. Because cosmic rays have energies up to a hundred million times those of the particles in the Large Hadron Collider, only something spectacularly violent could create them, scientists speculated.
Then, after interacting with other material in the jet, high-energy photons and neutrinos could be created from those cosmic rays.
Since cosmic rays are charged particles, it’s impossible to trace their paths to their origin, because magnetic fields affect and alter their paths. But neutrinos, while highly energetic, have no charge. Not even the most powerful magnetic field can affect them.
But how can there be two black holes? When galaxies that have a black hole at their center collide, sometimes it creates a black hole binary, or pair of black holes, at the center of the merged galaxies.
“It’s fantastic to understand the neutrino generation by studying the insides of jets,” Britzen said. “And it would be a breakthrough if our analysis had provided another candidate for a binary black hole jet source with two jets.”
The 2018 neutrino was found by sensors deep in the Antarctic ice in the IceCube detector.
The IceCube detector became operational at the South Pole in 2010. Largely funded by the National Science Foundation, as well as contributions from around the world, IceCube was built to detect high-energy neutrinos. It is the largest detector of its kind.
To build it, workers drilled 86 holes in the ice, each 1½ miles deep, and spread a network of 5,160 light sensors over a grid of 1 cubic kilometer. It’s operated by a team based at the University of Madison-Wisconsin, but the IceCube collaboration itself includes 300 scientists and 49 institutions.