Tiny particles deliver message from a hidden supermassive black hole

• "They’re interesting on both the largest and the smallest scales in nature," says Steven Prohira, a professor at the University of Kansas who studies ultra-high energy neutrinos.
• "Neutrinos are a bridge between what we know and what is just beyond what we understand."

How it works: The cosmic backdrop of neutrinos is thought to be mostly produced when cosmic rays — high-energy particles that travel through space at nearly the speed of light — smash into matter. Those collisions also create gamma rays, the highest-energy form of radiation.

• The chargeless, almost massless neutrinos aren't deflected by magnetic fields and interact weakly with matter, so if they hit Earth as they race through the universe they point back to their source.
• They're a unique messenger but detecting them is difficult. "It’s a really tough business — a heroic effort to do this," says Heino Falcke, a professor at Radboud University Nijmegen in the Netherlands who studies black holes.
• The IceCube neutrino detector at the South Pole uses more than 5,000 sensors buried 1 to 1.5 miles in the ice. When neutrinos hit molecules of ice they produce particles that emit blue light spotted by the sensors.
• Neutrinos and cosmic rays carry messages about their origins, which scientists have long sought. A leading candidate has been explosive gamma ray bursts.

Catch up quick: IceCube first detected an excess of 28 neutrinos from outside the Milky Way in 2013.

• In 2018, a blazar (TXS 0506+056) was identified as the point of origin of a single neutrino. Blazars are active galaxies with a supermassive black hole at the center that produces a jet of ionized matter.
• The team then went back through nearly 10 years of IceCube data and found evidence of neutrinos coming from other active galaxies.
• "A pattern emerged but we weren't sure whether we were seeing fluctuations in the data or if this was real," says Francis Halzen, principal investigator of IceCube and a professor at the University of Wisconsin-Madison.

What's new: A combination of improvements — in calibrating the detector, improving the data analysis, gaining a better understanding of the ice and the use of machine learning — has confirmed the pattern was real, Halzen says.

• In a study published today in the journal Science, the IceCube team reports evidence that the source of a stream of neutrinos detected between 2011 and 2020 can be traced back to the Messier 77 galaxy, or NGC 1068. (The statistical significance in the study isn't strong enough to qualify it as a discovery.)
• NGC 1068 is the second source of high-energy neutrinos found so far. The galaxy is about 47 million light-years from Earth.

The intrigue: At the center of NGC 1068 is an active galactic nucleus (AGN) fueled by a supermassive black hole hidden from Earth's view behind gas and dust.

• "Of course, for neutrinos, there is no problem," Halzen says. "They get out of everything."
• The researchers detected neutrinos but not gamma ray bursts from the AGN, "which now promotes them to the leading candidates for the sources of cosmic rays," he says, adding the production of the neutrinos may be very close to the black hole.

But, but, but... It isn't clear where exactly the neutrinos come from within the black hole system, says Falcke. "I see interesting debates in the future."

• Pinpointing that requires other information, including radio signals and a "comprehensive understanding of where and how particles are accelerated in a black hole," he says. "And neutrinos provide a new window to that."

The big picture: Neutrinos are one of four messengers scientists want to use to study large structures and events in the universe— like pulsars, black holes and the collisions between neutron stars. (The other messengers are cosmic rays, photons and gravitational waves.)

• Halzen said in a statement that the detection of these neutrinos is "the next big step towards the realization of neutrino astronomy."

What to watch: IceCube will get an upgrade in 2025 and more neutrino detectors are coming online, including KM3NeT in the Mediterranean Sea and the Baikal Gigaton Volume Detector in Russia.

• There are also plans to build instruments to try to detect ultra-high-energy neutrinos, including using radar, which is used by the Radar Echo Telescope that Prohira is developing.
• "One of the exciting things is how many complementary instruments there are," he says. They "point to an exciting future in the field."