Ever since French physicist Pierre Auger proposed in 1939 that cosmic rays must carry incredible amounts of energy, scientists have wondered what could produce those powerful clusters of protons and neutrons that rain down on Earth’s atmosphere. One possible way to identify such sources is to trace the paths taken by high-energy cosmic neutrinos on their way to Earth, since they are created by cosmic rays colliding with matter or radiation, producing particles that then decay into neutrinos and gamma rays.
Scientists at the IceCube neutrino observatory at the South Pole have now analyzed a decade of such neutrino detections and found evidence that an active galaxy called Messier 77 (aka Squid Galaxy) is a strong candidate for such a neutrino emitter at high energy, according to a new article 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 being able to really do neutrino astronomy,” MIT IceCube fellow Janet Conrad told APS Physics. “We have struggled for so long to see potential sources of very high importance cosmic neutrinos and now we have seen one. We have broken a barrier.”
As we have already pointed out, neutrinos move at a speed close to light. John Updike’s 1959 poem “Cosmic Gall” pays homage to the two most defining characteristics of neutrinos: they have no charge and, for decades, physicists believed they had no mass. (they actually have a very small mass). Neutrinos are the most abundant subatomic particle 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 our even noticing. This is why Isaac Asimov nicknamed them “ghost particles”.
This low rate of interaction makes neutrinos extremely difficult to detect, but because they are so light, they can escape unhindered (and therefore largely unaffected) through collisions with other particles of matter. This means they can provide valuable clues to astronomers about distant systems, further augmented by what can be learned with telescopes across the electromagnetic spectrum, as well as gravitational waves. Together, these different sources of information have been referred to as “multi-messenger” astronomy.
Most neutrino hunters bury their experiments deep underground, the better to cancel out noisy interference from other sources. In the case of IceCube, the collaboration includes arrays of basketball-sized optical sensors buried deep in Antarctic ice. On the rare occasions when a passing neutrino interacts with the nucleus of an atom in the ice, the collision produces charged particles that emit UV and blue photons. These are picked up by the sensors.
IceCube is therefore well placed to help scientists advance their knowledge of the origin of high-energy cosmic rays. As Natalie Wolchover convincingly explained to Quanta in 2021:
A cosmic ray is just an atomic nucleus, a proton or a group of protons and neutrons. Yet the rare cosmic rays known as “very high energy” cosmic rays have as much energy as tennis balls served by professionals. They are millions of times more energetic 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 detected, dubbed the “Oh-My-God Particle,” hit the sky in 1991 at something like 99.99999999999999999999951 percent of the speed of light, giving it at roughly the energy of a bowling ball dropped from shoulder height onto a toe.
But where do these powerful cosmic rays come from? A strong possibility is active galactic nuclei (AGNs), found at the center of some galaxies. Their energy comes from the supermassive black holes at the center of the galaxy and/or from the rotation of the black hole.
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