Cosmic rays are basically charged subatomic particles that streak to Earth from deep in outer space. Every once in a while, cosmic rays will have energies …
On September 22, 2017, a single neutrino traveling at nearly the speed of light zinged through Earth and set off IceCube’s detectors. It packed a whopping 290 TeV of energy—nearly 50 times more powerful than the most energetic proton beams running at the Large Hadron Collider. The hit triggered an alert notifying neutrino-hunting astronomers that the chase was on.
When scientists retraced this neutrino’s path, it led them to a spot on the sky near the constellation Orion, a location where, nearly simultaneously, several telescopes had spotted a massive cosmic flare.
Tucked into that patch of northern sky, a large, faraway object called a blazar had shaken itself awake and started hurling energetic particles into the void. That included a barrage of highly energetic gamma rays, which the space-based Fermi Gamma-Ray Space Telescope detected.
Those gamma rays came from a giant elliptical galaxy called TXS 0506+056 that contains a churning supermassive black hole at its core. As it snacks on nearby gas and dust, the black hole produces a jet of extremely energetic particles, which just happens to be pointed in Earth’s direction.
“Blazars are some of the most powerful astrophysical sources in the universe,” says Princeton University’s Maria Petropoulou. TXS 0506+056 is among the brightest blazars in the gamma-ray sky, which is remarkable given that it lives roughly four billion light-years away, and means that it is a prime candidate for producing very high-energy cosmic rays.
“It makes sense – it cannot be a wimpy blazar,” says Paolo Padovani of the European Southern Observatory. “If you’re going to see neutrinos, it has to be from a very, very powerful beast, otherwise you wouldn’t see them at all.”
In the days and weeks following the coincident IceCube and Fermi detections, multiple teams raced to study the flaring blazar. More than a dozen collaborations stared at the object in nearly all wavelengths of light, including radio, optical, x-rays and gamma rays, and it seemed that, indeed, a gamma-ray flare had erupted from TXS 0506+056 and produced IceCube’s September neutrino.