IceCube/NSF
Ever since French physicist Pierre Auger proposed in 1939 that cosmic rays should carry incredible amounts of energy, scientists have wondered what could produce these powerful clusters of protons and neutrons that rain down on Earth’s atmosphere. One possible way to identify such sources is to trace back the paths taken by high-energy cosmic neutrinos on their way to Earth, as they are created by cosmic rays colliding with matter or radiation, producing particles. which 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 (also known as 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 beginning of the possibility of really doing neutrino astronomy,” IceCube member Janet Conrad of MIT told APS Physics. “We’ve struggled for so long to see potential cosmic neutrino sources of very high significance and now we’ve seen one. We’ve broken a barrier.”
As we reported before, neutrinos travel at the speed of light. John Updike’s 1959 poem, “Cosmic Gall,” pays tribute to the two most defining features of neutrinos: They have no charge, and for decades physicists believed they had no mass (they actually have a tiny bit of mass). Neutrinos are the most abundant subatomic particle in the universe, but they rarely interact with any type of matter. We are constantly bombarded by millions of these tiny particles every second, but they pass right through us without us noticing. That’s why Isaac Asimov called them “ghost particles.”
Nicolle R. Fuller, IceCube/NSF
That low interaction speed makes neutrinos extremely difficult to detect, but because they are so light, they can escape unhindered (and therefore largely unchanged) through collisions with other matter particles. This means they can provide astronomers with valuable clues about distant systems, further complemented by what can be learned with telescopes across the electromagnetic spectrum, as well as gravitational waves. Together, these different sources of information have been termed “multitimesenger” astronomy.
Most neutrino hunters bury their experiments deep underground to eliminate noisy interference from other sources. In the case of IceCube, the collaboration involves arrays of optical sensors the size of a basketball, buried deep in 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 UV and blue photons. They are picked up by the sensors.
IceCube is thus well positioned to help scientists advance their knowledge of the origin of high-energy cosmic rays. As Natalie Wolchover convincingly explained at Quanta in 2021:
A cosmic ray is just an atomic nucleus – a proton or a cluster of protons and neutrons. Yet the rare ones known as “ultra-high-energy” cosmic rays have as much energy as professionally served tennis balls. They are millions of times more energetic than the protons that race through the circular tunnel of the Large Hadron Collider in Europe at 99.9999991% the speed of light. In fact, the most energetic cosmic ray ever detected, nicknamed the “Oh-My-God particle,” struck the sky in 1991 at a speed of about 99.99999999999999999999951 percent the speed of light, making it about the energy of a bowling ball that fell off the shoulder. height on one toe.
But where do such powerful cosmic rays come from? A strong possibility is active galactic nuclei (AGNs), which are found at the center of some galaxies. Their energy comes from supermassive black holes in the center of the galaxy and/or from the rotation of the black hole.
