IceCube neutrino analysis links possible galactic source of cosmic rays

Zoom / Artistic representation of a cosmic neutrino source shining over the IceCube Observatory in Antarctica. Under the ice are optical detectors that pick up neutrino signals.

IceCube / NSF

Ever since French physicist Pierre Auger suggested in 1939 that cosmic rays must carry huge amounts of energy, scientists have puzzled over what could produce these powerful clusters of protons and neutrons raining down on Earth’s atmosphere. One possible means of identifying such sources is to undo the paths high-energy cosmic neutrinos take on their way to Earth, since they arise from cosmic rays colliding with matter or radiation, resulting in particles that then decay into neutrinos and gamma rays.

Scientists at the IceCube Antarctic neutrino observatory have now analyzed a decade’s worth of neutrino discoveries and discovered evidence that an active galaxy called Messier 77 (also known as the Squid Galaxy) is a strong candidate for one of these high-energy neutrinos, according to a new research paper. Published in the journal Science. It brings astrophysicists one step closer to solving the mystery of the origin of high-energy cosmic rays.

“This observation marks the dawn of the ability to actually do neutrino astronomy,” Janet Conrad of MIT’s IceCube member told APS. “We’ve struggled for a long time to see potential cosmic neutrino sources of very high interest and now we’ve seen one. We’ve broken a barrier.”

As mentioned earlier, neutrinos move close to the speed of light. John Updike’s poem, “Cosmic Gall,” written in 1959, praises the two most important defining characteristics of neutrinos: they have no charge, and for decades, physicists have thought they have no mass (they actually have very little mass). Neutrinos are the most abundant subatomic particles in the universe, but they rarely interact with any type of matter. We are constantly bombarded every second by millions of these tiny particles, yet they pass through us without us noticing. That is why Isaac Asimov called them “ghost particles”.

When neutrinos interact with particles in clear Antarctic ice, they produce secondary particles that leave a trail of blue light as they travel through the IceCube detector.
Zoom / When neutrinos interact with particles in clear Antarctic ice, they produce secondary particles that leave a trail of blue light as they travel through the IceCube detector.

Nicole R. Fuller, IceCube/NSF

This low reaction rate makes neutrinos very difficult to detect, but because they are so light, they can escape unimpeded (and thus largely unaltered) by colliding with other matter particles. This means they could provide valuable clues to astronomers about distant systems, bolstered by what can be learned with telescopes across the electromagnetic spectrum, as well as gravitational waves. Together, these various sources of information have been called “Multiple Messenger” astronomy.

Most neutrino hunters bury their experiments deep in the earth, and it is better to cancel out loud interference from other sources. In the case of the IceCube, the collaboration features arrays of basketball-sized optical sensors buried deep in Antarctica’s ice. On those rare occasions when a transient neutrino interacts with the nucleus of an atom in the ice, the collision produces charged particles that emit ultraviolet light and blue photons. These are captured by sensors.

So IceCube is well positioned to help scientists advance their knowledge of the origin of high-energy cosmic rays. As Natalie Wilchover convincingly explained at Quanta in 2021:

A cosmic ray is just an atomic nucleus – a proton or a group of protons and neutrons. However, rare cosmic rays known as “ultra-energy cosmic rays” have just as much energy as professionally served tennis balls. They are millions of times more energetic than the protons orbiting around the circular tunnel of the Large Hadron Collider in Europe at 99.9999991% of the speed of light. In fact, the most energetic cosmic ray ever discovered, dubbed a “Oh my God particle,” hit the sky in 1991 at 99.9999999999999999999951 per cent of the speed of light, giving it the energy of a bowling ball that fell from shoulder height to toe height. .

But where do such powerful cosmic rays originate? One strong possibility is active galactic nuclei (AGNs), which are at the center of some galaxies. Its energy originates from the supermassive black holes at the center of the galaxy, and/or from the rotation of the black hole.

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