July 12, 2018: Neutrinos as messengers from a gigantic galaxy – another “falling wall”?

High-energy neutrino detected with IceCube on September 22, 2017. Credit: IceCube Collaboration
Christian Spiering, former spokesperson of IceCube, at the South Pole in 1996. Image courtesy of Christian Spiering.

Four billion years ago, our solar system and Earth still in their infancy, a neutrino was emitted in our direction by a huge galaxy with a supermassive black hole in its center. It eventually arrived at Earth on Sept. 22 last year and was recorded by our IceCube neutrino telescope at the South Pole. The energy of the neutrino was extremely high – 45 times higher than that of protons in CERN’s Large Hadron Collider. Coincident with that event, gamma-ray astronomers observed that this galaxy (named TXS 0506+056) was in a high state of activity. The probability that this coincidence was just accidental came out as only 1:1000. A clear proof that this galaxy is a neutrino source? Not yet: sometimes even things with small probability must happen! The really thrilling story followed when we scanned through all the data taken since 2009. For a period of few months in late 2014/early 2015, we observed a clear excess of a dozen neutrino events from the direction of TXS 0506+056. And the chance probability for such an excess is only 1:5000!

In my life, I have seen several walls falling, politically and scientifically. The fall of the Berlin wall in 1989 was clearly the most overwhelming event for me personally.  It happened just one year after I had started working in the field of neutrino astronomy. Next came the discovery of a diffuse flux of high-energy extraterrestrial neutrinos with IceCube in 2013, named the “breakthrough of the year” by the journal Physics World. “Diffuse” means that the arrival directions seemed to be spread uniformly over the whole sky, with no preferred direction indicating an individual source. And now the first evidence for exactly such an individual source!

Is this another “falling wall”? We don’t know yet for sure. The chance for such an observation just by accident is tiny, but it’s not below a millionth yet. This is the limit from where on the notoriously skeptical physicists call an observation confidently a “discovery”.

Neutrino physicists need stamina. The next activity outbreak of a galaxy might be observed very soon – and possibly be recorded not only by IceCube but also by neutrino telescopes just under construction. And only then, eventually, we will know that a new branch of astronomy has been born!  September 22, 2017 was just the exciting start.

Working at the South Pole

about the wonder of fresh breakfast eggs at one of the most remote outposts of civilization

PAX boarding LC-130 Hercules. © Gwen de Wasseige/ IceCube/ NSF
Amundsen-Scott South Pole Station. © Gwen de Wasseige/ IceCube/ NSF
Resting on the IceCube array. © Gwen de Wasseige/ IceCube/ NSF
IceCube laboratory. © Gwen de Wasseige/ IceCube/ NSF
Weather scroll. © NSF

For a physicist, having worked for years on improving the understanding of an experiment and its underlying models, it is actually a dramatic situation to be able to touch in situ the hardware that generates the data. Working on IceCube, situated at the Geographic South Pole, it is a particularly surrealistic adventure.

Depending on the weather, mechanical issues with the planes and scheduling constrains, the journey from Europe via New Zealand and the Antarctic logistics hub McMurdo can take anywhere from four days to two weeks. Once at “the Pole”, you feel struck by the strong contrast created of  a fossil fuel-driven outpost of civilization (single rooms, spacious laboratories & recreational rooms and three warm meals a day) clashing with the harsh conditions of one of the coldest, driest and most remote places on Earth. The absurdity of this situation is probably best exemplified by the cult-like status surrounding fresh breakfast eggs and the sadness that ensues when none are available after a week without flights.

IceCube is a unique experiment using the deep glacial ice which has accumulated over the last 100 millennia and it’s outstanding optical properties as a detection medium for a particle physics experiment. With the over 5000 sensors frozen about 2km deep in the ice, the only reminders of the experiment are small islands of bamboo flags marking the location of each drill hole as well as a small server-room building in the centre of the instrumented area. As such it takes quite a leap of imagination to appreciate that one is standing on top of nearly 3km of ice, with about 3000 particles being detected and stored per second.

After working on “the ice” for three weeks, we are now heading home. This is a moment comprising an ambivalent mixture of feelings: The anticipation to come home, the sadness to leave this remarkable place and the anxiety as the small R&D telescope we deployed will hopefully survive the grueling winter to gather the required data.