First neutrino image of an active galaxy

Prof. Dr. Elisa Risconi

Photo: Prof. Dr. Elisa Risconi
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Credit: Andreas Heddergott / TUM

For more than ten years, the IceCube Observatory at the South Pole has been observing the light effects of extragalactic neutrinos. While evaluating observatory data, an international research team led by the Technical University of Munich (TUM) discovered a source of high-energy neutrino radiation in the active galaxy NGC 1068, also known as Messier 77.

The universe is full of secrets. One of these enigmas involves active galaxies with supermassive black holes at their centres. “Today we still don’t know exactly what processes are going on there,” says Elisa Risconi, professor of experimental physics with cosmic particles at TUM. Her team has now taken a big step toward solving this mystery: astrophysicists have discovered a high-energy neutrino source in spiral galaxy NGC 1068.

It is very difficult to explore the centers of active galaxies with telescopes that detect visible light, gamma rays, or X-rays from space, because clouds of cosmic dust and hot plasma absorb radiation. Only neutrinos can escape from hell at the edges of black holes. These neutrinos have no electric charge and almost no mass. They permeate space without being deflected by electromagnetic fields or being absorbed. This makes it very difficult to detect.

The biggest hurdle in neutrino astronomy to date has been separating the very weak signal from the strong background noise caused by the effects of particles from Earth’s atmosphere. It took many years of measurements using the IceCube Neutrino Observatory and new statistical methods to enable Resconi and her team to piece together enough neutrino events for their discovery.

Detective work in Eternal Ice

The IceCube telescope, located in Antarctic ice, has been detecting traces of light from fallen neutrinos since 2011. “Based on their energy and angle of incidence, we can reconstruct where they came from,” says TUM scientist, Dr. Theo. Glush. “The statistical evaluation shows a very important set of neutrino effects coming from the direction of the active galaxy NGC 1068. This means that we can assume with near certainty that the high-energy neutrino radiation comes from this galaxy.”

The spiral galaxy, 47 million light-years away, was discovered as early as the 18th century. NGC 1068 – also known as Messier 77 – is similar in shape and size to our galaxy, but its center is very bright and is much brighter than the entire Milky Way, although the center is roughly the size of our solar system. This center contains an “active core”: each black supernova with a mass about one hundred million times the mass of our Sun, which absorbs large amounts of material.

But how and where are neutrinos generated there? “We have a clear scenario,” Risconi says. “We think that high-energy neutrinos are the result of the intense acceleration of matter near the black hole, which raises it to very high energies. We know from particle accelerators experiments that high-energy protons generate neutrinos when they collide with other particles, in other words: we found an accelerator cosmic.

Neutrino observatories for new astronomy

NGC 1068 is the most statistically significant source of high-energy neutrinos discovered to date. More data will be necessary in order to be able to localize and investigate weaker and farther neutrino sources, says Risconi, who recently launched an international initiative to build a neutrino telescope with a volume of several cubic kilometers in the northeastern and Pacific Ocean. The neutrino experiment, P-ONE. Together with a planned second generation IceCube observatory – IceCube Gen2 – it will provide data for future neutrino astronomy.

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