High-energy cosmic neutrinos traced back to their home quasars
There are billions of tiny particles called neutrinos streaming through your body right now. But where did they come from? Russian researchers have now traced back some ultra-high energy neutrinos to their points of origin – radio flares from raging quasars.
Neutrinos are elementary particles with a neutral charge and almost no mass at all. They’re extremely common, produced in nuclear reactors and weapons, the Sun, supernovae, and cosmic rays interacting with the Earth’s atmosphere.
But some of them are accelerated to extremely high energies in deep space, and how and where this occurs remains mostly a mystery. Finding out was the goal of the new study, by researchers at the Moscow Institute of Physics and Technology (MIPT), the Russian Academy of Sciences and the Max Planck Institute.
Despite being very common, neutrinos are hard to study. That’s because they don’t interact much with other matter, passing effortlessly through entire planets. But every now and then, one will randomly interact with another particle, and specialized detectors can pick this signal up.
To protect them from outside interference, neutrino detectors are usually built deep underground, underwater or under ice sheets. That way, other particles will deflect off the molecules at or near the surface, while neutrinos will sail right on through.
In this case, the team used data from IceCube, a neutrino observatory buried under 2 km (1.2 mi) of ice in Antarctica. They were looking for extremely high-energy neutrinos, of 200 trillion electron volts (Tera-electron volts, TeV) or more.
Their hypothesis was that high-energy neutrinos are produced during radio flares from quasars – galaxies with particularly active black holes at their centers. As matter falls towards the black hole, protons are sometimes accelerated and ejected out into space, becoming the energetic neutrinos.
While these have been detected coming from quasars before, it was thought that they always accompanied gamma ray bursts. The team on the new study suspected they might instead be produced during radio flares.
To find out, the researchers analyzed 50 neutrino events picked up by IceCube, and compared the data to measurements of quasars from radio telescopes around the world. In particular, they used the Russian radio telescope RATAN.
And sure enough, they discovered that at the same time as the neutrinos were hitting Earth, radio flares with frequencies of over 10 GHz were often recorded coming from certain quasars.
“At first the results seemed ‘too good’ to be true, but after carefully reanalyzing the data, we confirmed that the neutrino events were clearly associated with the signals picked up by radio telescopes,” says Sergey Troitsky, an author of the study. “We checked that association based on the data of years-long observations of the RATAN telescope of the RAS Special Astrophysical Observatory, and the probability of the results being random is only 0.2%. This is quite a success for neutrino astrophysics, and our discovery now calls for theoretical explanations.”
The team now plans to investigate how exactly quasars produce neutrinos, and recheck the original findings.
The research was published in the Astrophysical Journal.