The study of astronomy has evolved since ancient times, when people observed the skies relying only on what they could see with the naked eye. Telescopes built by the famous Galileo Galilei, known as the father of observational astronomy, provided a means of gaining an even better look at the universe using visible light, but the universe is vast and sometimes opaque in regions, so no matter how accurate the telescope, there are limits to what can be observed in visible light.
The 20th century saw astronomy advance in leaps and bounds. New experimental devices were developed that enabled astronomers to observe the sky in electromagnetic wavelengths other than those of visible light, such as X-rays and radio waves. In addition, we are also seeing developments in observational astronomy utilizing messengers that are not part of the electromagnetic spectrum at all. This includes neutrinos and gravitational waves, observations of which were thought extremely challenging until only a short time ago.
The linking of data gathered by observatories looking at different targets using the various “messengers” is known as “multi-messenger astronomy,” and it helps researchers to unravel the mysteries of the universe.
Electromagnetic and gravitational waves, neutrinos, and cosmic rays are all generated using different mechanisms, which means that neutrino sources can be identified by combining the observational data gathered using different astronomical observational tools and correlating that data to time and the neutrino’s direction of travel.
The IceCube group also harnesses multi-messenger astronomy. We developed a real-time alert system that accurately analyzes important data about cosmic neutrino events (the Cherenkov radiation signal generated by charged particles from neutrino collisions) detected by IceCube in real time, then delivers it to astronomical observatories around the world. The system was brought online in April 2016.
The IceCube group at Chiba University was responsible for development of the Extremely High Energy (EHE) track event selection for alerts corresponding to cosmic neutrino detection sensitivity in the energy band from 100TeV to 1EeV.
Most of the neutrino events detected by IceCube are background events induced from the interaction of atoms in the atmosphere, and they are not as important. The system algorithms select which events contain important information and are therefore good candidates for analysis. Only the highest-priority data is analyzed in real-time, then an alert is issued to observatories around the world containing information on the event, including direction. Telescopes at each observatory receiving an alert are turned to look towards the origin of the neutrino and determine whether there are astronomical objects in that direction displaying any unusual activity.
Alerts are sent out about 30 seconds after a neutrino event is detected, but observation may not be possible if it is daytime at the receiving observatory or if weather is poor.
Timing is also important because observations will be conducted as night falls in each location and when observing conditions improve.
Four alerts were issued over approximately one year after the first EHE neutrino event alert was sent in July 2016, but even though gamma-ray and other telescopes followed up their observations to trace back the neutrino paths, no successful identification was made.
On 23 September 2017 at 05:54 Japan Standard Time (or 22 September 2017 at 20:54 UTC), a high-energy neutrino event was identified by IceCube’s EHE track selection. It was a typical high-energy track event travelling horizontally across the IceCube detection volume and was detected under favorable conditions that allowed accurate estimation of the neutrino source direction.
Direction of travel and other observational data on IC170922A was delivered by alert to observatories around the world and tracking of the potential source began.
Hiroshima University’s KANATA Telescope received the alert and began observations 20 hours after the neutrino event was detected and discovered that the blazar TXS0506+056 – which is found in the direction of the neutrino’s path – was brighter in the visible light region. Based on that discovery, Hiroshima University researchers working with the Fermi Gamma-ray Space Telescope began analyzing observational data from the Fermi LAT collaboration and found that the object was emitting gamma rays at a much higher intensity than normal.
TXS0506+056 is a known blazar (of the BL-Lac type) with a massive black hole at its core, and it was already listed in the Fermi LAT Source Catalogue. The blazar had been active since April 2017 and was emitting gamma rays at roughly six times its normal intensity. The IC170922A neutrino event was detected during that period of active gamma-ray emission.
Another system of telescopes called MAGIC, or the Major Atmospheric Gamma Imaging Cherenkov Telescopes, is sensitive to higher-energy gamma rays, and it detected gamma rays with very high energies exceeding 100 GeV from this object. This was the first time that very-high-energy gamma rays were observed in the same period and same direction as a high-energy neutrino event detected by IceCube.
Subsequently, tests were conducted to ensure that IceCube’s neutrino observation and the increased brightness of the gamma rays were not simply a coincidence. Testing showed that the result was significant by 4σ. That means that the chance the two events were unrelated coincidences was only 0.003%. In addition, researchers reviewed past IceCube data and subjected the data from that direction to a more rigorous analysis and confirmed that a large number of neutrino events from the direction of the same object were detected during a 160-day period around 16 December 2014. This confirmation meant that it could be proved that this was the first time in history that researchers had identified a source of neutrino radiation.
The results were published in the US journal Science in the two following articles.
Title of Paper：Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A
Author(s)：The IceCube, Fermi-LAT, MAGIC, Kanata, Kiso teams et al.
Title of Paper：Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert
This research served as a major step forward in understanding the still unresolved mysteries surrounding the mechanism behind extremely high energy cosmic rays, and it attracted attention from around the world. It was selected as one of the top ten breakthroughs of the year in 2018 by Science magazine.
The world’s first detection of a neutrino source showed the effectiveness of IceCube’s Realtime Alert System and the techniques of multi-messenger astronomy.
In the future, we aim to fortify this system of observation, engage in even closer cooperation with observatories around the world, and identify even more such astronomical objects.
In addition, it is essential to increase neutrino sensitivity of our detectors – as neutrinos are the source of our tracking observation information – and to increase the number of event detections. Using IceCube Upgrade (scheduled to start in 2022) and the later IceCube-Gen2 programs, we will increase our detection volume to roughly eight times that of the current IceCube array in order to detect more neutrino events, enabling us to identify even more astronomical sources of neutrino emission.
Learn more about IceCube Upgrade and IceCube-Gen2 here.