IceCube detected neutrinos from a nearby galaxy

The 1 km3 detector buried in Antarctic ice has identified a second neutrino source outside the Milky Way. This is a nearby active galaxy that opens up the catalogue of potential targets for neutrino-based astronomy.

The IceCube scientific collaboration, which operates a 1 km3 detector buried in the ice of the South Pole, has formally discovered a second source of neutrinos located outside the Milky Way galaxy, confirming that the most elusive particles in the Universe can, despite the extreme difficulty of detecting them, be used for astronomical observation. According to an article published in the journal Science on 3 November, the creation site of the neutrinos in question is at the heart of the galaxy NGC 1068, where a super-massive black hole obscured by huge clouds of gas and dust is present. The discovery owes much to the preliminary work of one of the main authors of the study, Teresa Montaruli, a professor in the Department of Nuclear and Corpuscular Physics (Faculty of Science) and founding member of the IceCube project. Two years earlier, she and her doctoral student at the time, Tessa Carver, had already identified this galaxy by processing ten years of data produced by IceCube. However, the precision of the result obtained at the time was insufficient to call it a 'discovery', whereas the study published in Sciences'a comes much closer to this fateful threshold.

Neutrinos are the most ghostly particles in the universe. They have no electric charge and their mass, although not zero, is very small. They are sensitive to only one of the four forces of nature, the weak force, whose range is not much larger than the radius of the atomic nucleus. In other words, once a neutrino is created, there is almost nothing to stop it. Of the 65 billion neutrinos (of all types) that 'hit' every second the smallest square centimetre of the Earth, only a few collide by chance with atomic nuclei right in their path.

It is this property of near-invisibility that interests astrophysicists. Since they are not absorbed by dust clouds, unlike grains of light such as photons, nor are they deflected from their trajectory by magnetic fields, as charged particles would be, high-energy neutrinos, created at cosmological distances in supernovae or at the edges of black holes, reach the Earth in a straight line. They are therefore extremely valuable potential direct messengers from the deep universe.

Setup of the IceCube detector buried in the South Pole ice. Image: IceCube Collaboration

The IceCube detector was built just a stone's throw from the South Pole to intercept them. The device consists of 86 2.5 km deep boreholes dug into the ice. A 'collar' of 60 detectors, or photomultipliers, was inserted into each well. The work lasted five years and, since December 2010, the 5160 'digital optical modules', which fill a volume of 1 km3 , have been recording the passage of neutrinos that are willing to leave a trace.

Ice is an interesting medium for hunting these elusive particles. It is transparent and, at this depth, very dark. This is crucial because when a cosmic neutrino encounters an ice atom, it usually produces a special elementary particle, a muon. This muon, a kind of large ephemeral electron, has a very high speed right from the start, exceeding the speed of light in ice (not in a vacuum). It then emits a characteristic blue light (known as Cherenkov light), which is precisely what the IceCube detectors pick up.

What is this blazar?

The overwhelming majority of neutrinos (or rather muons) measured by IceCube are produced by cosmic rays colliding with atoms in the atmosphere. Astrophysicists must therefore subtract this background noise to retain only the particles from the deep Universe. This corresponds to an average of about ten events detected per year.

The first image obtained from these very high-energy particles, whose extragalactic origin has been demonstrated, is a flow coming equally from all regions of space. From there, the challenge was to identify the types of point sources that could be responsible for this diffuse signal. 

The first IceCube trophy is TXS 0605+056, a 'blazar' discovered as a neutron point source by a team that also included Teresa Montaruli. It is a galaxy with an active nucleus, driven by a super-massive black hole. The latter, which is a large devourer of matter, has formed an accretion disc around itself and generates a jet of very hot gas and radiation along its axis of rotation. What is special about the blazar is that its jet is directed right at the Earth and produces neutrino outbursts whenever it collides with clouds of matter. On 27 September 2017, IceCube recorded such an outburst, triggering an immediate alert from the scientific community. In the aftermath, other ground- and space-based observatories turned their attention to the suspected source and were able to confirm the existence of a signal through observations of cosmic rays, in the gamma-ray, X-ray, visible light and radio wave domains. This is one of the first examples of 'multiple messenger astronomy'.

Galaxy NGC 1068, also known as Meissier 77. At its centre is a source of neutrinos detected by IceCube. Image: HSP/NASA/ESA

NGC 1068 is therefore the second capture of the giant detector. Although it shares some commonalities with the first, this active galaxy, also known as Messier 77, has some fundamental differences, expanding the catalogue of objects potentially detectable by IceCube. 

Located 47 million light-years away, the galaxy NGC 1068 also hosts a super-massive black hole (one of the closest to us apart from the one at the centre of the Milky Way), surrounded by a disc of accretion matter," explains Teresa Montaruli. The difference with TXS 0506+056 is that the jet of NGC 1068 is much weaker and, above all, is not directed towards the Earth at all. Another peculiarity is that NGC 1068 contains regions of very intense star formation. These continuously forming stars die in explosions that cause winds and shocks that result in accelerating particles."

Astronomers believe that the neutrinos detected by IceCube are produced by the high-energy encounter between the jet and the part of the accretion disk that lies very close to the black hole's horizon and/or between the jet and the dust clouds surrounding the black hole and the stellar winds produced by the continuously forming stars in this active galaxy. 

DegrEE OF certainty limit

The formal discovery of the neutrino source in NGC 1068 took time. Given the small number of neutrinos that are collected (about 80 in all for NGC 1068), it took years for a signal to begin to appear and become statistically significant. 

The first results, from Tessa Carver's thesis (which was awarded the J. Wurth Prize for the best thesis in 2019) and published on 6 February 2020 in the journal Physical Review Letters, are based on data from the first ten years of IceCube. They already reveal a fairly strong signal with a certainty level of 2.9, which corresponds to a confidence interval of just under 99.73%. The problem is that, in this field of astrophysics, a discovery is only a discovery from 5, i.e. a probability of being wrong of 1 in 3.5 million. The ultimate goal was therefore missed, but the result was enough to trigger numerous studies and the development of models by groups around the world. 

The study published in Science benefited from a complete recalibration of all IceCube detectors and two more years of measurements. Thanks to this, and using a data processing similar to that developed by Teresa Montaruli's team for the first publication, the scientists were able to obtain a certainty level of 4.2, which brings them one step closer to the discovery threshold.

In the future, the IceCube collaboration (which brings together some 60 universities from around the world and is supported by a dozen national scientific research agencies) will not only add more years of data to make new discoveries. Instrument upgrades, planned for 2025 or 2026, will improve the angular resolution of the detector under the ice. Above all, the plan is to build its successor, IceCube-Gen2, which will be 10 times larger and will, in fact, be the world's largest particle detector. It will be installed at the same location and the work should last until at least 2032.