Ghost particle may solve mystery of high energy cosmic rays
Thursday, July 12, 2018, 11:50 AM - After a journey of 4 billion years, a tiny ghost particle may have supplied astronomers with the solution to a persistent mystery of science - the source of high energy cosmic rays.
For over 100 years, astronomers have been trying to locate an elusive source of super high-energy particles, known as cosmic rays, that they have been detecting from space.
According to two new papers, published in the journal Science, the detection of a single tiny 'ghost particle', using a unique Antarctic observatory called IceCube, as well as followup observations from telescopes around the world and in space, may have finally pinpointed this mystery source.
In this artistic rendering, a blazar is accelerating protons that produce pions, which produce neutrinos and gamma rays. Neutrinos are always the result of a hadronic reaction such as the one displayed here. Gamma rays can be produced in both hadronic and electromagnetic interactions. Credit: IceCube/NASA
Back on September 22, 2017, the IceCube Neutrino Observatory, buried under more than a kilometre of Antarctic glacier ice, recorded multiple flashes of light through its array of detectors. This detection, called IC170922, was identified as being from an extremely high energy neutrino, a so-called 'ghost particle'. Using the path the particle took as it passed through the observatory, astronomers backtracked its trajectory through space, and pointed Earth and space telescopes in that direction.
What they focused in on was one particular celestial object, some 4 billion light years away, just off the shoulder of Orion, which goes by the rather cumbersome name of TXS 0506+056.
"Tracking that high-energy neutrino detected by IceCube back to TXS 0506+056 makes this the first time we’ve been able to identify a specific object as the probable source of such a high-energy neutrino," said Gregory Sivakoff, of the University of Alberta, according to the National Radio Astronomy Observatory.
TXS 0506+056 is a 'blazar' - an active, rotating supermassive black hole, nestled in the core of a distant galaxy, which is emitting intense magnetic fields as it spins in space, and producing twin jets of highly energetic particles as a result.
This illustration depicts a blazar - an active black hole, with its accretion disk and twin jets of high-energy charged particles, streaming away into space from its poles. Credit: Sophia Dagnello, NRAO/AUI/NSF
It was thought that even this extreme of object couldn't explain the most energetic cosmic rays astronomers were detecting, though.
"It is interesting that there was a general consensus in the astrophysics community that blazars were unlikely to be sources of cosmic rays, and here we are," Francis Halzen, the lead scientist for the IceCube Neutrino Observatory, from the University of Wisconsin-Madison, said in a statement on Thursday.
The black hole is active due to its crushing gravity pulling in surrounding matter, which then forms into a bright disk of material around it, called an accretion disk. As its powerful magnetic fields rotate through the accretion disk, charged particles are picked up and carried towards the magnetic poles, where they concentrate and are accelerated into intense particle beams that are emitted out into space. In essence, the black hole acts as a natural particle accelerator in space, but at much higher energies than our artificial accelerators, here on Earth, can generate.
Now, the charged particles in these beams are the same kind that are detected here at Earth as cosmic rays - mostly positively-charged protons and atomic nuclei - but it's highly unlikely that we would actually see cosmic rays directly from TXS 0506+056 show up here at Earth, event though one of the particle beams from the blazar is pointed directly at us.
This is because - as charged particles - these cosmic rays generate magnetic fields as they move through space, and thus their path can be altered by any other magnetic field they encounter on their journey, such as those generated by stars and galaxies. Since even tiny changes in course can result in immense differences in destination over billions of years of time, there's no way that we could trust that any particular high energy cosmic ray detected here actually came from this blazar.
Astronomers hit upon a bit of luck, however.
As Halzen explains: "Now, we have identified at least one source that produces high-energy cosmic rays, because it produces cosmic neutrinos. Neutrinos are the decay products of pions. In order to produce them you need a proton accelerator."
Neutrinos are subatomic particles that are so small, they only rarely interact with any other matter on their journey through the universe. At this very moment, and during each second of every day, roughly 10 trillion neutrinos are passing through our bodies, and every bit of matter surrounding us. We simply don't feel them (and they have no idea we exist), because they slip right past or through all of our atoms and molecules without encountering anything in their way. They can even pass right through the Earth without stopping.
On occasion, however, simply by chance, one of these neutrinos will have a head-on collision with an atom or molecule. The more matter you put in a neutrino's path, and the denser that matter is, the greater the chance of a collision occurring.
The trick, though, is getting the neutrino to collide with something, and have a detector available that can actually see and record the collision.
Antarctica's IceCube Neutrino Observatory is nearly perfect for the job.
The IceCube Neutrino Observatory encompasses a cubic kilometer of pristine ice deep below Antarctica's surface and next to the NSF Amundsen-Scott South Pole Station. In this illustration, based on an aerial view near the South Pole, an artistic rendering of the IceCube detector shows the interaction of a neutrino with a molecule of ice. The display pattern is how scientists represent data on recorded light. Every colored circle represents light collected by one of the IceCube sensors. The color gradient, from red to green/blue, shows the time sequence. Credit: IceCube Collaboration/NSF
The observatory consists of a 3-dimensional array of extremely sensitive light sensors, evenly spread throughout a giant cube of dense glacier ice, a kilometre on a side, located roughly 1.5 km below the surface of the Antarctic ice sheet.
The density of the ice and the size of the array not only maximizes the potential for a collision, but the pristine, clear ice at that depth allows the array sensors to pick up the tiny flashes of blue light, known as Cherenkov radiation, that are produced by each neutrino collision. Plotting the data recorded by each detector in the array reveals the path of the neutrino in three dimensions, and that path can be traced back to the source.
This is high-energy neutrino IC170922, as detected by IceCube on Sept. 22, 2017. The neutrino event display shows a muon, created by the interaction of a neutrino with the ice very close to IceCube, which leaves a track of light while crossing the detector. In this display, the light collected by each sensor is shown with a colored sphere. The color gradient, from red to green/blue, show the time sequence. Credit: IceCube Collaboration
Tracing neutrino IC170922 back along its trajectory was only the first clue to this mystery, however. The astronomers had to determine how a blazar, which was pretty far down on the list of potential sources of high energy neutrinos and cosmic rays, could actually produce this neutrino.
For the next steps, they turned to other telescopes, both on Earth and above it, to investigate TXS 0506+056 - in as many wavelengths of light as possible - for a possible 'smoking gun'.
Since this blazar has been observed for some time already, due to it being a known source of high energy gamma rays, the researchers delved back into the records from a number of different telescopes, hoping to match up the detection of this neutrino with any other detections from this source.
On Sept. 22, 2017, IceCube alerted the international astronomy community about the detection of a high-energy neutrino. About 20 observatories on Earth and in space made follow-up observations, which allowed identification of what scientists deem to be a source of very high energy neutrinos and, thus, of cosmic rays. Besides neutrinos, the observations made across the electromagnetic spectrum included gamma-rays, X-rays, and optical and radio radiation. These observatories are run by international teams with a total of more than one thousand scientists supported by funding agencies in countries around the world. Credit: Nicolle R. Fuller/NSF/IceCube
What they found was an intense burst of light from TXS 0506+056 - the strongest gamma-ray flare detected from this source in a decade of observations from the Fermi telescope - which coincided with the arrival of neutrino IC170922.
Other telescopes also recorded observations of this event, in different wavelengths of light, with the Earth-sized radio observatory known as the Very Long Baseline Array actually detecting 'knots' of intense radio emission inside the stream. According to the researchers, these bright knots are likely regions of higher density material inside the particle beam, emitting strong bursts of energy.
"The behavior we saw with the VLA is consistent with the emission of at least one of these knots. It’s an intriguing possibility that such knots may be associated with generating high-energy cosmic rays and thus the kind of high-energy neutrino that IceCube found," Sivakoff said, according to the NRAO.
This gathering of data from multiple observations of one event, at different wavelengths, all lending support to a particular discovery, is an example of 'multi-messenger astronomy'. The detection of gravitational waves, which were accompanied by observations of different emissions of light from the source, is another example of this 'multi-messenger astronomy'.
WHY IS THIS SO IMPORTANT?
So, astronomers and scientists are understandably excited about this discovery, but what difference does it make? Why is it so important to know the source of high energy neutrinos and cosmic rays?
It comes down to answering some of the bizarre questions that have arisen as we have explored the universe so far. In this case, specifically, the question is - how is it that nature can produce events of such intense energy, that they are far beyond the scale of our understanding.
More generally, though, where does a large part of the energy of the known universe come from, what does that mean about the structure of the universe, and how does that impact our place in this universe?
Dr. Francis Halzen explains below:
With so much going on at TXS 0506+056, telescopes continue to observe it, and IceCube will no doubt be looking for more neutrinos from this source.
"There are a lot of exciting phenomena going on in this object," said Sivakoff.