An international team of scientists has found first evidence of high-energy neutrino source: a flaring active galaxy, or blazar, 4 billion light years from Earth.
Following a detection by the IceCube Neutrino Observatory in September 2017, the European Space Agency’s (ESA) INTEGRAL satellite joined a collaboration of observatories in space and on the ground that kept an eye on the high-energy neutrino source, heralding the thrilling future of multi-messenger astronomy.
Neutrinos are nearly massless, ‘ghostly’ particles that travel essentially unhindered through space at close to the speed of light. Despite being some of the most abundant particles in the Universe, with almost 100 000 billion passing through our bodies every second, these electrically neutral, subatomic particles are notoriously difficult to detect because they interact with matter incredibly rarely.
What is the difference between neutrinos and cosmic rays?
Most neutrinos arriving at Earth derive from the Sun, but those that reach us with the highest energies are thought to stem from the same sources as cosmic rays.
Unlike neutrinos, cosmic rays are charged particles so their path is bent by magnetic fields. The neutral charge of neutrinos instead means they are unaffected by magnetic fields and because they pass almost entirely through matter, they can be used to trace a straight path all the way back to their source.
Acting as ‘messengers’, neutrinos directly carry astronomical information from the far reaches of the Universe. Over the past decades, several instruments have been built on Earth and in space to decode their messages. However, the source of high-energy neutrinos has remained unproven.
The IceCube Observatory
The IceCube observatory, detects neutrinos through their secondary particles, muons. These muons are produced on the rare occasion that a neutrino interacts with matter near the detector, creating tracks as they pass through layers of Antarctic ice.
Their long paths mean their position can be well defined, and the source of the parent neutrino can be pinned down in the sky.
During the event in September, a traversing muon deposited 22 TeV of energy in the IceCube detector. Scientists estimated the energy of the parent neutrino to be around 290 TeV, indicating a 50% chance that it had an astrophysical origin beyond the Solar System.
When the origin of a neutrino couldn’t be identified by IceCube the researchers sent their finding to an international network of observatories.
These included NASA’s Fermi gamma-ray space telescope and the Major Atmospheric Gamma-Ray Imaging Cherenkov (MAGIC) on La Palma, in the Canary Islands, which found the known blazar, TXS 0506+056, in a ‘flaring’ state – a period of intense high-energy emission – at the same time the neutrino was detected at the South Pole.
What are blazars?
Blazars are the central cores of giant galaxies that host an actively accreting supermassive black-hole at their heart, where matter spiralling in forms a hot, rotating disc that generates enormous amounts of energy, along with a pair of relativistic jets.
These jets are colossal columns that funnel radiation, photons and particles – including neutrinos and cosmic rays. A specific feature of blazars is that one of these jets happens to point towards Earth, making its emission appear exceptionally bright.
ESA’s INTEGRAL gamma-ray observatory was part of this international collaboration.
Carlo Ferrigno from the INTEGRAL Science Data Centre at the University of Geneva, Switzerland said: “This is a very important milestone to understanding how high-energy neutrinos are produced,
“There have been previous claims that blazar flares were associated with the production of neutrinos, but this, the first confirmation, is absolutely fundamental. This is an exciting period for astrophysics.”
Did INTEGRAL record the high-energy neutrino source?
INTEGRAL, which surveys the sky in hard X-rays and soft gamma rays, is also sensitive to transient high-energy sources across the whole sky. At the time the neutrino was detected, it did not record any burst of gamma rays from the location of the blazar, so scientists were able to rule out prompt emissions from certain sources, such as a gamma-ray burst.
The fact that INTEGRAL could not detect the source provided significant information about this blazar, allowing scientists to place a useful upper limit on its energy output during this period.
Scientists now have the capability to detect a plethora of ‘cosmic messengers’ travelling vast distances at extremely high speeds, in the form of light, neutrinos, cosmic rays, and even gravitational waves.
For more details about the high-energy neutrino source, visit ESA.