The tracing of the source of high-energy neutrinos signals a significant development in the use of ‘multi-messenger’ astronomy.
The source of high-energy ‘cosmic neutrinos’ has eluded scientists for decades, that was until last September when such a particle struck a detector buried in ice at the South-Pole, research published in Science reveals. The event was coupled with the detection of a flaring ‘blazar’ by NASA’s Fermi Gamma-Ray Telescope giving us a clue as to the origin of high-energy neutrinos. This discovery is not just significant for our knowledge of these particles however, it may help usher in a whole new age of ‘multi-messenger’ astronomy.
Why do high-energy neutrinos matter?
One of the most staggering achievements of modern physics is the discovery of one of nature’s fundamental particles, the neutrino. Whilst the discovery of other particles has been aided by the benefit of their intrinsic mass or charge, the neutrino is virtually massless and has no charge. In fact, they have such little effect on the world around them that 65 billion of them stream through every square centimetre of your body every second completely unnoticed. The only interaction they leave is when they strike an atomic nucleus.
The most common form of neutrinos that reach Earth originate from the Sun, with other neutrino sources being traced to distant supernovae. But the Earth is also exposed to high-energy neutrinos which are far less common. This rarity may explain why science has had such difficulty detecting the origin of such particles.
A ‘cosmic neutrino’ is defined as a neutrino carrying with it so much energy that it must have originated outside our solar system. In 2013 the first two such particles, nicknamed ‘Bert’ and ‘Ernie’ were detected by the South-pole based IceCube neutrino experiment, a cube of ice with more than 5,000 detectors embedded into it, manned by 300 scientists from across 12 countries.
Since 2013, the IceCube has been detecting roughly 8 high-energy neutrinos per year, sending out a global alert to astronomers upon the finding of such a particle, in the hope that their source can be discovered. But until now such a discovery has eluded the global scientific community.
What was different about the latest detection of a high-energy ‘cosmic’ neutrino?
On September 22nd, a high-energy neutrino stuck within a cubic Kilometer of the South-Pole based IceCube neutrino experiment. The neutrino was followed by a muon which left a light trail which allowed researchers to trace the neutrino back to the location of its source in the night sky.
As usual, the detection of the neutrino sparked a global-alert and the race was on amongst researchers again to locate the source of this ‘cosmic’ neutrino. This time the search was not fruitless.
Astronomers scoured the area of the sky indicated by IceCube eventually finding, via NASA’s Fermi Large Area Gamma-Ray Telescope, the likely source of the cosmic ray in which the particles travelled, a flaring blazar 3.7 billion light-years from Earth. A blazar is an active galactic nucleus, from which a relativistic jet of particles created by the black hole at the centre of the galaxy ripping apart in-falling material is directed towards Earth.
As well as the Fermi telescope, another space-based gamma-ray telescope named SWIFT and the ground-based telescope MAGIC in the Canary Isles also detected gamma-ray bursts and other surges from the same source. Collaboration between institutes continued and eventually, 18 observatories had confirmed the flares in various wavelengths of the electromagnetic spectrum at the spot from which the neutrino and muon had been traced.
These twin detections, of the high-energy neutrino and the observation of the flaring blazar to the west of Bellatrix, a star in the constellation Orion, named TXS 0506+056 were first linked together by Yasuyuki Tanaka of Hiroshima University. But this alone was not enough for researchers to draw conclusions about the source of high-energy neutrinos, working on the hints given by the finding they set about scouring previously recorded data to strengthen the connection.
Researchers at IceCube discovered that this area of the night sky was quite special, it had been extremely active in the production of high-energy neutrinos for four or five months at the end of 2014 and the beginning of 2015. They linked the production of these neutrinos with the emission of high-energy gamma rays, finding that when the neutrinos had less energy the gamma-rays produced were more energetic and vice-versa.
This allowed the researchers to conclude that blazars are indeed the source of high-energy ‘cosmic’ neutrinos, making them only the third known source of neutrinos after stars such as the Sun and supernovae.
What does ‘multi-messenger’ astronomy mean for the future of science?
This is the second huge result in what scientists refer to as ‘multi-messenger’ astronomy, the first being the detection of gravitational waves produced by two clashing neutron stars by LIGO in August 2017.
Multi-messenger astronomy refers to the practice of using more than one signal to detect astronomical objects. So in the example documented above for instance, astronomers used the detection of neutrinos in conjunction with the observation of gamma-ray flares to detect a blazar.
Whereas in the past astronomy may have been limited to what could be detected by visible light, this then expanded to various other areas of the electromagnetic spectrum such as gamma-rays, x-rays and radio waves. The ability to ‘see’ astronomical objects has now expanded beyond the electromagnetic spectrum at all with the use of gravitational waves and neutrinos becoming a real possibility.
Multi-messenger astronomy uses two or more of these methods to observe astronomical objects, allowing us to build a picture of the universe and observe events that early astronomers could only dream of.
The reason this is so superior to methods that rely on the electromagnetic spectrum alone is that neutrinos and gravitational waves are unaffected by magnetic fields they may encounter on their epic, universe-spanning journey to Earth, due to their lack of an electric charge.
The fact that neutrinos travel across the universe unimpeded means that none of the information they carry from their source is lost on their journey. Astronomers hope that this means the observation of neutrinos will allow them to build a picture of their source more accurately than the observation of electromagnetic radiation allows.
Eventually, such observations may allow astronomers to trace the origins of ultra-high-energy cosmic rays, streams of heavy particles that rain down to Earth created by unknown events in the universe, that physicists speculate may share a source with high-energy neutrinos. This will only become possible when science is able to detect high-energy neutrinos in far greater numbers of course.
Thus the discovery of at least one of the sources of such particles may well aid their detection in the future.