The IceCube observatory detects neutrino and discovers a blazar as its source
Posted by admin on 12th July 2018

About four billion years ago, when the planet Earth was still in its infancy, the axis of a black hole about one billion times more massive than the sun happened to be pointing right to where our planet was going to be on September 22, 2017.

Blazar shoots neutrinos and gamma rays to Earth: Blazars are a type of active galactic nucleus with one of its jets pointing toward us. In this artistic rendering, a blazar emits both neutrinos and gamma rays that could be detected by the IceCube Neutrino Observatory as well as by other telescopes on Earth and in space.

Along the axis, a high-energy jet of particles sent photons and neutrinos racing in our direction at or near the speed of light. The IceCube Neutrino Observatory at the South Pole detected one of these subatomic particles – the IceCube-170922A neutrino – and traced it back to a small patch of sky in the constellation Orion and pinpointed the cosmic source: a flaring black hole the size of a billion suns, 3.7 billion light years from Earth, known as blazar TXS 0506+056. Blazars have been known about for some time. What wasn’t clear was that they could produce high-energy neutrinos. Even more exciting was such neutrinos had never before been traced to its source.

Finding the cosmic source of high-energy neutrinos for the first time, announced on July 12, 2018 by the National Science Foundation, marks the dawn of a new era of neutrino astronomy. Pursued in fits and starts since 1976, when pioneering physicists first tried to build a large-scale high-energy neutrino detector off the Hawaiian coast, IceCube’s discovery marks the triumphant conclusion of a long and difficult campaign by many hundreds of scientists and engineers – and simultaneously the birth of a completely new branch of astronomy.

The constellation of Orion, with a bullseye on the location of the blazar.
Silvia Bravo Gallart/ Project_WIPAC_Communications, CC BY-ND

The detection of two distinct astronomical messengers -– neutrinos and light –- is a powerful demonstration of how so-called multimessenger astronomy can provide the leverage we need to identify and understand some of the most energetic phenomena in the universe. Since its discovery as a neutrino source less than a year ago, blazar TXS 0506+056 has been the subject of intensive scrutiny. Its associated stream of neutrinos continues to provide deep insights into the physical processes at work near the black hole and its powerful jet of particles and radiation, beamed almost directly toward Earth from its location just off the shoulder of Orion.

As three scientists in a global team of physicists and astronomers involved in this remarkable discovery, we were drawn to participate in this experiment for its sheer audacity, for the physical and emotional challenge of working long shifts at in a brutally cold location while inserting expensive, sensitive equipment into holes drilled 1.5 miles deep in the ice and making it all work. And, of course, for the thrilling opportunity to be the first people to peer into a brand new kind of telescope and see what it reveals about the heavens.

A remote, frigid neutrino detector

At an altitude exceeding 9,000 feet and with average summertime temperatures rarely breaking a frigid -30 Celsius, the South Pole may not strike you as the ideal place to do anything, aside from bragging about visiting a place that is so sunny and bright you need sunscreen for your nostrils. On the other hand, once you realize that the altitude is due to a thick coat of ultrapure ice made from several hundred thousand years of pristine snowfall and that the low temperatures have kept it all nicely frozen, then it might not surprise you that for neutrino telescope builders, the scientific advantages outweigh the forbidding environment. The South Pole is now the home of the world’s largest neutrino detector, IceCube.

March 2015: The IceCube Laboratory at the Amundsen-Scott South Pole Station, in Antarctica, hosts the computers that collect raw data from the detector. Due to satellite bandwidth allocations, the first level of reconstruction and event filtering happens in near real time in this lab.
Erik Beiser, IceCube/NSF

It may seem odd that we need such an elaborate detector given that about 100 billion of these fundamental particles sashay right through your thumbnail each second and glide effortlessly through the entire Earth without interacting with a single earthly atom.

In fact, neutrinos are the second most ubiquitous particles, second only to the cosmic microwave background photons left over from the Big Bang. They comprise one-quarter of known fundamental particles. Yet, because they barely interact with other matter, they are arguably the least well understood.

To catch a handful of these elusive particles, and to discover their sources, physicists need big – kilometer-wide – detectors made of an optically clear material – like ice. Fortunately Mother Nature provided this pristine slab of clear ice where we could build our detector.

The IceCube Neutrino Observatory instruments a volume of roughly one cubic kilometer of clear Antarctic ice with 5,160 digital optical modules (DOMs) at depths between 1,450 and 2,450 meters. The observatory includes a densely instrumented subdetector, DeepCore, and a surface air shower array, IceTop.
Felipe Pedreros, IceCube/NSF

At the South Pole several hundred scientists and engineers have constructed and deployed over 5,000 individual photosensors in 86 separate 1.5-mile-deep holes melted in the polar ice cap with a custom-designed hot-water drill. Over the course of seven austral summer seasons we installed all the sensors. The IceCube array was fully installed in early 2011 and has been taking data continuously since.

This array of ice-bound detectors can sense with great precision when a neutrino flies through and interacts with a few Earthly particles that generate dim patterns of bluish Cherenkov light, given off when charged particles move through a medium like ice at close to light speed.