Blue light in Antarctic ice helps illuminate violent black hole

A tiny particle’s dramatic death at the South Pole spawns one of the year’s biggest science discoveries

Credit: Nicolle R. Fuller/NSF/IceCube

Last year, on the first day of the Antarctic spring, a silent traveler from a galaxy nearly four billion light-years away crashed into the Earth. It was a neutrino, one of the tiniest particles in the universe, and its journey ended when it struck the core of a single water molecule roughly a kilometer deep within the Antarctic ice sheet.

The collision released a subatomic particle called a muon, traveling in a straight line from the impact. Because the muon travels faster in ice than the surrounding light, a blue shockwave of Cherenkov radiation emerged. The illuminated cone revealed that a collision occurred and the direction from which it originated.

Credit: Nicolle R. Fuller/NSF

No one was around to see the blue light erupt, or countless other strikes that emerge in the ice every year. The reason scientists know the collision took place is because it occurred within a massive detector buried deep beneath the South Pole.

The NSF IceCube Neutrino Observatory at NSF’s Amundsen-Scott South Pole Station encompasses a cubic kilometer of pristine ice embedded with glass sensors tuned to detect exactly that sort of light. It is a precision instrument that took years to build and is overseen by hundreds of scientists from across the globe.

Credit: Nicolle R. Fuller/NSF/IceCube

On Sept. 22, at exactly 20:54:30.43 Coordinated Universal Time, the detector witnessed the death of a neutrino traveling with tremendous energy, flagging its source as something extremely powerful.

That the particle struck Earth is not unusual, as countless neutrinos strike our planet and us every moment of every day. The reason we don’t notice them is that they are extremely tiny and only interact weakly with matter, which is why it helps to have a detector that can watch a square kilometer of material.

Credit: IceCube Collaboration

What made this neutrino special is that scientists were able to catch the collision as it happened and recognize that it had been travelling at an extremely high energy (almost three hundred billion electron volts). Researchers then alerted telescopes across the globe to scan the sky to try to locate its source.

And they did.

Nearly two dozen observatories — from NSF’s ground-based Very Large Array in New Mexico to NASA’s Fermi satellite in space — were able to trace the neutrino’s path back to a violent galaxy core called a blazar.

Credit: Nicolle R. Fuller/NSF

Blazars are enormous black holes that actively consume stars and solar systems only to spit the consumed matter and energy out in massive, opposing jets — and this one had a jet pointed directly at Earth.

The blazar — TXS 0506+056 — is the first object beyond our sun that astrophysicists can prove produces neutrinos. With a full suite of telescopes capturing the source across a range of light wavelengths and new neutrino data to analyze, astronomers expect to learn entirely new science about blazars and other powerful objects in the sky.

The discovery is the latest example of a new approach to studying the cosmos, where telescopes tracking photons join observatories tracking other “messengers,” such as gravitational waves detected by NSF’s Laser Interferometer Gravitational-wave Observatory (LIGO) or the neutrino particles picked up by NSF’s IceCube.

This new era of multi-messenger astrophysics is the target for one of NSF’s 10 Big Ideas for Future NSF Investments, and it has only just begun.

For the full story, and insights into how the discovery reveals blazars as one of the long-hunted sources of cosmic rays, see “Neutrino observation points to one source of high-energy cosmic rays.”