Ever since French physicist Pierre Auger proposed in 1939 that cosmic rays must carry incredible amounts of energy, scientists have puzzled over what could produce these powerful swarms of protons and neutrons that rain down into Earth’s atmosphere. One possible way to detect such sources is to retrace the paths traveled by high-energy cosmic neutrinos on their way to Earth, as they are created by cosmic rays colliding with matter or radiation, producing particles that then decay into neutrinos and gamma rays .
Scientists with 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 the Squid Galaxy) is a strong candidate for such a high-energy neutrino emitter, according to a new 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 actually being able to do neutrino astronomy,” IceCube member Janet Conrad of MIT told APS Physics. “We’ve struggled so long to see possible cosmic sources of neutrinos at a very large scale and now we’ve seen one. We’ve broken a barrier.”
As we have mentioned before, neutrinos travel at close to the speed of light. John Updike’s 1959 poem, “Cosmic Gall,” pays tribute 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 small mass). Neutrinos are the most abundant subatomic particle in the universe, but they very rarely interact with any kind of matter. We are constantly bombarded every second by millions of these tiny particles, but they pass through us without us even noticing. That’s why Isaac Asimov called them “phantom particles.”
This low interaction rate makes neutrinos extremely difficult to detect, but because they are so light, they can escape unhindered (and thus largely unchanged) from collisions with other matter particles. This means they can provide astronomers with valuable clues 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 called “multiple message” 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 involves basketball-sized arrays of optical sensors buried deep within the Antarctic ice. On those rare occasions when a passing neutrino interacts with the nucleus of an atom in the ice, the collision produces charged particles that emit ultraviolet and blue photons. These are collected by the sensors.
Thus, IceCube is well positioned to help scientists advance their understanding of the origin of high-energy cosmic rays. As Natalie Wolchover persuasively explained at Quanta in 2021:
A cosmic ray is simply an atomic nucleus – a proton or a cluster of protons and neutrons. However, the rare ones known as “ultra-high energy” cosmic rays have as much energy as professional tennis balls. They are millions of times more energetic than the protons spinning 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, nicknamed the “Oh-My-God particle,” hit the sky in 1991 at 99.999999999999999999999951 percent of the speed of light, giving it about the energy of a falling ball . height at toe.
But where do such powerful cosmic rays come from? A strong possibility is active galactic nuclei (AGNs), located at the center of some galaxies. Their energy comes from supermassive black holes at the center of the galaxy and/or from the rotation of the black hole.
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