Till the early part of last century, astronomy was traditionally associated with the detection of light from celestial sources. Astronomy with other forms of light - photons - arrived on the scene in the 1930s with radio astronomy and infrared, X-ray, Gamma ray following it in the next few decades. However, the particle which has eventually competed with photons in solving the mystery of the cosmos is the neutrino, which was predicted in the 1930s as a possible solution to an academic puzzle.
Since then, we have a deeper understanding of the particle world in which neutrino is recognised as one of the fundamental particles of nature called leptons. Additionally, three types of neutrinos - electron neutrino, muon neutrino and tau neutrino - have also been discovered. Neutrinos are neutral particles and have very little interaction with other forms of matter. These can traverse distances of continents without any or little interaction. This necessitates that detectors have to be huge to record any neutrinos.
Cosmic ray origin
The first role of neutrinos in astrophysics was in understanding the sun and the stars. The neutrinos, which are also emitted during nuclear fusion, were detected by giant-sized experiments in the last quarter of the 20th century. Known as solar neutrinos, their detection proved that fusion is indeed the reaction in stars. Another great discovery was of neutrino emission along with light at the end of a heavier star's evolution culminating in a supernova. These neutrinos were discovered in February 1987 when a supernova - SN 1987A - appeared in a nearby galaxy. These and other experiments detected a strange property of neutrinos called neutrino oscillation in which one type of neutrinos would change to other type and vice versa.
Apart from these, there are also artificial neutrinos from nuclear reactors and particle accelerators. There are neutrinos coming from geothermal activity in earth's interior. While these neutrinos are of lower energy (MeV), cosmic ray neutrinos (the results of several interactions of cosmic ray particles in the atmosphere) are 1,000 to 10,000 times higher than solar neutrinos. These are called atmospheric neutrinos and were first detected in the Kolar Gold Fields, Karnataka. Neutrinos are so ubiquitous that trillions of solar neutrinos and 1,000 atmospheric neutrino pass through our body every second.
However, the highest energy neutrinos from TeV to PeV energies are expected to arise in processes inside powerful astrophysical sources like quasars, blazars and radio galaxies. These are collectively known as active galactic nuclei. These harbour supermassive black holes at the centre and can accelerate particles like protons to very high energies.
When these particles stream out they interact with the ambient matter to produce both photons and neutrinos. However, the photons cannot travel long distances because of absorption due to various processes. It is only neutrinos that can pass unimpeded through interstellar and intergalactic space without getting disturbed. A study of these neutrinos, termed astrophysical neutrinos, can lead to the solution of the cosmic ray origin problem.
There have been several attempts in the past to study these astrophysical neutrinos. Since detectors have to be very big, it was suggested in the 1970s that huge waterbodies would provide the necessary target for the neutrino to interact. The ensuing particle after the interaction would produce a different type of light called Cerenkov radiation in water.
This could be detected by light sensitive detectors like photomultipliers placed deep so as to minimise the noise background due to neutrinos from atmospheric muons and neutrinos. An example of this is ANTARES, a detector that is placed 2.5 km under the Mediterranean Sea off the coast of France. However, detectors in pure ice were proposed in the late 1980s since water has several sources of background light like bioluminescence.
Ice as the medium was also preferred since Cerenkov light travels greater than 100 metres in ice without much scatter. Based on this principle, a detector array called AMANDA was constructed in Antarctica earlier and was later replaced with a much bigger detector called IceCube Neutrino Observatory. The observatory, which started working seven years ago, encompasses a cubic kilometre of ice in Antarctica.
The basic detector is a cable containing 60 photosensitive devices embedded in a hole in the ice at levels greater than 1.5 km from the surface, the depth assuring that most of the noise particles are filtered out. There are 80 such cables spread over a total area of one sq km. Incoming neutrino interacts giving out muon which gives out Cerenkov radiation while traversing the ice.
This information, picked up by the optical detectors, helps in constructing the path of the parent neutrino and eventually to the point of its origin in the space. The angular accuracy is of the order of a degree. Till today, the experiment has detected more than 80 high-energy neutrinos (60 TeV to 10 PeV), which have been confirmed unambiguously to be of astrophysical origin. This amounted to picking a needle in a haystack since it had to be isolated from a sample which involved more than a million atmospheric neutrinos and hundreds of billions of cosmic-ray muons.
However, the events do not show any correlation with the following: the position of known celestial sources either in the galaxy or outside the galaxy; no known gamma ray emitting any active galactic nuclei; the powerful events like Gamma Ray bursts; or the gravitational wave sources detected in the last two years. This rather large flux of seemingly unassociated neutrinos is a mystery and can only be attributed to a possible diffuse flux at the moment.
The other major study at the IceCube Neutrino Observatory is of particle physics. Recently, the experiment detected fewer energetic neutrinos which completely pass through the earth without getting absorbed. This agrees very well with the Standard Model prediction that the probability that a neutrino interacts with matter increases with energy. With many experiments concurrently happening on neutrinos, we may be able to get deeper insights into the nature of neutrinos.