A huge neutrino observatory buried deep in the ice of Antarctica has discovered only the second extra-galactic source of the elusive particles ever found.
In results published last week in Science, the IceCube collaboration reports the detection of neutrinos from an “active galaxy” called NGC 1068, located some 47 million light-years from Earth.
How to spot a neutrino
Neutrinos are very shy fundamental particles that often don’t interact with anything else. When they were first detected in the 1950s, physicists soon realized that they would in some ways be ideal for astronomy.
Because neutrinos so rarely have anything to do with other particles, they can travel unhindered through the universe. However, their shyness also makes them difficult to detect. Catching enough to be useful requires a very large detector.
That’s where IceCube comes in. Over seven summers, from 2005 to 2011, scientists at the US Amundsen–Scott South Pole Station drilled 86 holes in the ice with a hot water drill. Each hole is nearly 2.5 kilometers deep, about 60 centimeters wide and contains 60 basketball-sized light detectors attached to a long cable.
How does this help us detect neutrinos? Sometimes a neutrino hits a proton or neutron in the ice near a detector. The collision produces a much heavier particle called a muon, traveling so fast that it emits a blue glow, which light detectors can pick up.
By measuring when this light arrives at different detectors, the direction the muons (and neutrinos) are coming from can be calculated. Looking at the particle energies, it turns out that most of the neutrinos detected by IceCube are created in the Earth’s atmosphere.
However, a small fraction of neutrinos come from outer space. In 2022, thousands of neutrinos from somewhere in the distant universe have been identified.
Where do neutrinos come from?
They seem to come fairly evenly from all directions, with no obvious bright spots appearing. That means there must be plenty of neutrino sources out there.
But what are these sources? There are many candidates, exotic-sounding objects like active galaxies, quasars, blazars, and gamma-ray bursts.
In 2018, IceCube announced the discovery of the first identified high-energy neutrino emitter: a blazar, which is a special type of galaxy that shoots a jet of high-energy particles towards Earth.
Known as TXS 0506+056, the blazar was identified after IceCube saw a single high-energy neutrino and sent an urgent Astronomer Telegram. Other telescopes rushed to take a look at TXS 0506+056 and found that it was also emitting a lot of gamma rays at the same time.
This makes sense, because we think blazars work by boosting protons to extreme speeds, and these high-energy protons then interact with other gases and radiation to produce both gamma rays and neutrinos.
An active galaxy
The blazar was the first extra-galactic source ever discovered. In this new study, IceCube has identified the second.
IceCube scientists re-examined the first decade of data they had collected, applying sophisticated new methods to obtain more accurate measurements of neutrino directions and energy.
As a result, an already interesting bright spot in the neutrino background glow has become sharper. About 80 neutrinos came from a fairly nearby and well-studied galaxy called NGC 1068 (also known as M77, as it is the 77th entry in the famous 18th century catalog of interesting astronomical objects created by the astronomer French Charles Messier).
Located about 47 million light-years from Earth, NGC 1068 is a known “active galaxy,” a galaxy with an extremely bright core. It’s about 100 times closer than blazar TXS 0506+056, and its angle to us means that gamma rays from its core are obscured from our view by dust. However, the neutrinos are happily hurtling through the dust and into space.
This new discovery will provide a wealth of information to astrophysicists and astronomers about what exactly is going on inside NGC 1068. There are already hundreds of papers trying to explain how the inner core of the galaxy works, and the new IceCube data adds neutrino information that will help refine these models.
This article is republished from The conversation under Creative Commons license. Read the original article.
Image Credit: NASA / ESA / A. van der Hoeven
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