Neutrinos from a nearby galaxy reveal the secrets of a black hole

Neutrinos from a nearby galaxy reveal the secrets of a black hole

In the zoo of subatomic particles, neutrinos are strange beasts. Unlike more familiar particles such as electrons and protons, ghostly neutrinos barely interact with normal matter: they can fly through a planet as if it isn’t even there. This makes them extremely difficult to detect and, for neutrinos from cosmic objects in the sky, even harder to know exactly where they came from. However, in recent research published in Science, an extragalactic source for these subatomic particles has been identified.

For the first time, astronomers have confidently detected neutrinos from NGC 1068, a galaxy with a massive, actively feeding black hole at its center. Neutrinos are created above the black hole’s “point of no return” – its event horizon – although it’s unclear exactly how; several mechanisms are plausible. Scientists hope this discovery will change their understanding of not just NGC 1068, but everything such galaxies. As a bonus, they believe the discovery may have revealed the source of a faint neutrino glow we see everywhere we look in the sky.

Matter falling toward a black hole first forms a flattened accretion disk orbiting around it. Friction heats this disc of matter to incredible temperatures, causing it to glow so brightly that it eclipses the entire host galaxy. We call these “active galaxies” and they are among the brightest objects in the universe.

In the case of NGC 1068, however, it is difficult to detect this bright light because thick clouds of opaque cosmic dust absorb almost all of it, leaving virtually no signal out. This is where the most annoying property of neutrinos is an advantage for us: they can break through those dust clouds and fly off into space, eventually reaching Earth. Yet we are left with the problem of detecting them. How do you measure neutrinos when they pass through your detector unscathed? The good news is that, for neutrinos, matter is only most of them transparent. Although only in extraordinarily rare cases do some manage to interact with matter, but it takes a very special type of observatory to see it.

Located almost exactly at the south pole of the Earth, the IceCube neutrino observatory is such a place, and it is do not your standard astronomical setup. For one thing, it doesn’t use a mirror to collect and focus light from cosmic objects like telescopes do; instead, it has a series of relatively simple optical sensors strung along dozens of vertical strings, creating a 3D network of over 5,000 sensors capable of detecting the location and times of flashes of light.

On the other hand, it is buried under more than a kilometer of Antarctic ice. When a neutrino passes through ice, it has a small chance of hitting the nucleus of one of the oxygen or hydrogen atoms in that ice. A real impact is extremely rare: billions of neutrinos pass through every cubic centimeter of matter on Earth. each second, but measurable physical interactions with this material can only occur every few days.

When they do, however, it creates high-velocity subatomic shrapnel, particles traveling away from the nuclear collision site at slightly less than the speed of light. These also pass through the ice. Here’s the fun part: they actually travel faster than the speed of light can travel through ice.

However, no laws of physics are broken. The speed of light in the void is the ultimate cosmic speed limit, but the speed of light is slower as it passes through matter. Particles cannot travel faster than light in a vacuum, but they can travel faster than light through matter. When they do, they create a sort of photonic sonic boom, like the shock wave created when something moves through the air faster than the speed of sound. These faster-than-light events manifest as bright flashes of blue light called Cherenkov radiation. They can be seen from some distance through the clear ice of Antarctica and picked up by IceCube’s sensors.

This technique allows scientists to detect neutrino events from space, although there is a problem with unwanted events that mimic the real signals. Subatomic particles from other sources in the universe called cosmic rays can hit our atmosphere and create similar flashes of light, confusing measurements. Scientists can, however, differentiate between the two types of signals in a clever way: using the Earth itself as a huge filter. Neutrinos from space will come from all directions, including through the Earth. Cosmic rays, however, will only come from the sky above the Antarctic Observatory because they cannot pass directly through Earth like neutrinos do. IceCube’s detectors can measure direction and filter out events coming from above, ensuring scientists only retain the impacts of cosmic neutrinos.

IceCube has detected millions of such events from neutrinos, many of which appear to originate from sources scattered randomly and evenly across the sky. Somewhere out there in the universe are legions of neutrino sources. The question is, what are they?

Looking at data collected from 2011 to 2020, the IceCube collaboration – a huge collection of scientists, engineers, data analysts, etc. – handled every detected event very carefully. Using directional information from the flashes to trace the paths of incoming cosmic neutrinos, they found several points in the sky that appeared to be statistically significant neutrino sources.

The detection with the largest number of neutrinos? A total of 79 (plus or minus about 20) neutrinos over this period from the direction of NGC 1068.

This beautiful spiral galaxy is relatively close, just 47 million light-years away, and bright enough to spot with binoculars. Previous work analyzing neutrinos from IceCube indicated that NGC 1068 was a possible source, but the data was not strong enough at the time to claim a discovery. These new results are a game-changer.

Detecting neutrinos ostensibly from this active galaxy is a big problem. The neutrinos that astronomers have seen have tremendous energy, greater than one tera-electron-volt each. That’s trillions of times the energy of visible light photons that we see coming from the galaxy. The enormous particle energy must be created in an extremely powerful cosmic particle accelerator, and with a large, actively fueling black hole, several options are available.

For example, the turbulent ionized miasma of matter above and below the disk of matter around the black hole is infernally hot and contains powerful magnetic fields that can pump vast energies into the particles, accelerating them to near speeds of the light. Another way involves the magnetic field in this accretion disk twisting near the black hole, creating twin tornado-like vortices, called jets, which can also hurl particles at high speeds; the shock waves generated in the jets when charged particles collide can also generate the energies needed to create high-energy neutrinos. Such jets are known to exist in NGC 1068.

Detecting these neutrinos from NGC 1068 will give astronomers insight into the forces involved and the specific engines responsible for them – a boon given the hidden nature of black holes.

And although only a few dozen NGC 1068 neutrinos have been detected on Earth, this number is diluted by the great distance they have traveled in the vast volume of space. Given this reduction, astronomers calculate that the total number of neutrinos generated by the black hole must be so huge that they take away 10 billion times more energy than the sun emits.

These sightings also provide a major clue to another mystery. Neutrinos arrive at Earth from all over the sky, creating a background glow across the skies. The source of this glow has been difficult to pinpoint. However, neutrinos from several other active galaxies have also been observed in the IceCube data (although with less statistical certainty than NGC 1068). There are several million of these galaxies spread throughout the universe. The new data indicates that, if they emit neutrinos much like NGC 1068 does, these more distant galaxies could be the source of the cosmic neutrino background, just as individual stars in the sky coalesce to form the continuous glow of the Milky Way which you can see from a dark site at night.

Not so long ago, we knew of only two sources of astronomical neutrinos: the sun, where neutrinos are created in nuclear fires from its core, and supernova 1987A, a relatively nearby explosive star that emitted a flash neutrino transient once and then was gone.

Every major galaxy in the universe has a supermassive black hole in its core, and any of them can potentially be active. But, although ubiquitous, they can be difficult to observe. With a positive detection of neutrinos from at least one and probably more, astronomers have opened a new window into these prodigious monsters.

This is an opinion and analytical article, and the opinions expressed by the author or authors are not necessarily those of American scientist.

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