Around four billion light years away, just to the left of the constellation Orion, an active galactic nucleus is blasting ultrahigh-energy cosmic rays and neutrinos right at us. This jet of particles and energy is the by-product of that galaxy’s central black hole shredding and consuming captured objects. As intense as such a stream of energy may be, this particular nucleus, named TXS 0506+056, appears even brighter than usual because it’s aimed straight at Earth, qualifying it as a blazar. The weird part of all this is that even though astronomers have long been able to detect the energy from this blazar across all bands of the electromagnetic spectrum, we’ve only just been able to trace the stream of particles it sends out for the first time.
Tracking the paths of particles
The two types of particles blasting out of TXS 0506+056 are cosmic rays and high-energy neutrinos, and both carry their own challenges for detection. The cosmic rays are mostly super-charged protons, which are known to be flying all around the universe. They can and do collide with other particles which makes their detection a bit easier, but because they are positively charged particles, their paths get bent and reshaped by every magnetic field they fly through. As such, even when you detect a cosmic ray, it’s nearly impossible to know where it came from on.
Neutrinos can help solve that problem, but not without creating challenges of their own. These tiny particles are sort of a neutral version of a proton, generally created when protons have been smashed and accelerated in the same conditions that would create cosmic rays. As their name implies, they are electromagnetically neutral and thus can fly through the cosmos without interacting with magnetic fields. This means that if they’re detected, you can count on them to have been moving in a straight line, creating a traceable path back to their source. However, detecting them is very difficult, because they’re so small, fast and neutral that they can fly through objects without interacting with anything. To really try to figure out where in space these super-fast particles, scientists had to build a very specialized detector in the ice of Antarctica.
Illumination in the ice
The IceCube Neutrino Observatory at the Amundsen-Scott South Pole Station has turned a cubic kilometer of ice into the world’s best neutrino detector. Over 5,000 light sensors have been embedded in the ice in a grid formation. When a neutrino does happen to collide with atoms in the clear ice, it releases a cone of blue light that the detectors can then measure. By measuring which detectors see the brightest light, the original trajectory of the neutrino can be pieced together. After checking to eliminate collisions from slower neutrinos created by comic rays hitting Earth’s atmosphere, coordination begins with astronomers around the world to verify the high-energy neutrino’s suspected point of origin.
On September 22, 2017, a collision was detected in the ice that had the signature of galaxy-crossing, high-energy neutrino. An alert was then sent out to various telescopes around the world as quickly as possible so that they could look for any cosmic activity that could explain the neutrino’s trajectory. NASA’s orbiting Fermi Gamma-ray Space Telescope and the Major Atmospheric Gamma Imaging Cherenkov Telescope, or MAGIC, converged on the aforementioned TXS 0506+056 blazar. Researchers then looked at data from previous years to see if this trajectory was part of a possible pattern, and were able to find more instances of neutrino and gamma ray activity that also shared that point of origin. More data is obviously desired to really confirm these findings, but right now this all strongly suggests that we finally traced a high-energy neutrino back to its source.
A whole new way to know what’s in space
The idea that a blazar like TXS 0506+056 could pump out neutrinos and cosmic rays isn’t a shock. The energy necessary to get a neutrino and comic ray to be traveling close to the speed of light isn’t easy to come by, and something as violent as a blazar or galaxy collision would fit the bill. Learning more about such an object is obviously exciting, but the real significance of this detection is how it opens up a new way to study space and astrophysics. For the most part, everything we know about the universe outside our solar system has depended on measuring some portion of the electromagnetic spectrum, from radio waves to light to gamma rays. That energy can’t always be found though, and so tracking neutrinos gives us a whole new way to collect data on the universe. Coupled with the recent detection of gravity waves, scientists are comparing this so-called ‘multi-messenger’ detection to suddenly developing a new sensory organ in your body. We’ve long been able to see into space, but now we’ve gained a new way to touch it as well.
My fourth-grader asked: So if a neutrino can go through big objects without hitting them, can they pass through a black hole?
As the IceCube detector demonstrates, neutrinos can hit other atoms, but their speed, size and lack of charge helps them avoid doing so most of the time. None of those things offer a way to avoid the bent space-time of a black hole though, which can famously capture light which moves faster and has less mass than a neutrino. So while a black hole can help produce neutrinos and spray them into space, none of those neutrinos would be going anywhere if they’re aimed at the black hole itself.
Source: More than century-old riddle resolved—a blazar is a source of high-energy neutrinos by University of Wisconsin-Madison, Phys.org