IceCube / NSF
Since French physicist Pierre Auger suggested in 1939 these cosmic rays must carry huge amounts of energy, scientists have wondered what could produce these powerful clusters of protons and neutrons raining down on Earth’s atmosphere. One possible way to identify such sources is to trace the paths that high-energy cosmic neutrinos take on their way to Earth, where they are created by cosmic rays colliding with matter or radiation, resulting in particles that then decay into neutrinos and gamma rays.
Scientists with an ice cube The Antarctic Neutrino Observatory has now analyzed a decade of these neutrino discoveries and found evidence that an active galaxy called Messier 77 (also known as the Squid Galaxy) is a strong candidate for such a high-energy neutrino emitter, according to a new paper published in Science magazine. It brings astrophysicists one step closer to solving the mystery of the origin of high-energy cosmic rays.
“This observation represents the dawn of the ability to actually do neutrino astronomy,” said Janet Conrad, an Ice Cube Fellow from MIT. Says APS Physics. “We have struggled for a long time to see the possible sources of cosmic neutrinos of very high importance and now we have seen one. We have broken a barrier.”
As we’ve already reported, neutrinos travel near the speed of light. John Updike’s 1959 poem, “Cosmic Bitterness” commemorates the two most defining features of neutrinos: they have no charge, and for decades physicists have believed they have no mass (in fact they have very little mass). Neutrinos are the most abundant subatomic particles in the universe, but they very rarely interact with any type of matter. We are constantly bombarded every second by millions of these tiny particles, but they pass through us without even noticing them, which is why Isaac Asimov called them “ghost particles”.
Nicole R. Fuller, IceCube/NSF
This low rate of interaction makes neutrinos extremely difficult to detect, but because they are so light, they can escape unhindered (and thus largely unaffected) from collisions 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 referred to as “multi-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 includes arrays of basketball-sized optical sensors buried deep in Antarctica’s ice. On the rare occasions that a transient neutrino interacts with the nucleus of an atom in the ice, the collision produces charged particles that emit ultraviolet and blue photons. These are captured by the sensors.
So IceCube is well positioned to help scientists advance their knowledge of the origin of high-energy cosmic rays. As Natalie Wilchover convinced and explained to 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 “extremely high energy” cosmic rays have the same amount of energy as tennis balls served by professionals. They are millions of times more powerful than the protons hurtling 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% of the speed of light, giving it roughly the energy of a bowling ball that fell from shoulder height to a toe. .
But where do these powerful cosmic rays come from? There is a strong possibility of active galactic nuclei (AGNs), found in the center of some galaxies. Its energy comes from supermassive black holes at the center of the galaxy and/or from the black hole’s rotation.
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