TRACING THE SOURCE OF COSMIC RAYS
Astronomers said the discovery could provide a clue to one of the enduring mysteries of physics and the cosmos, writes
IT was the smallest bullet you could possibly imagine, a subatomic particle weighing barely more than a thought. It had been fired out of a gravitational gun barrel by a cosmic blunderbuss, a super-massive black hole.
On Sept. 22, 2017, a particle known as a neutrino zinged down from the sky and through the ice of Antarctica at nearly the speed of light, setting off a cascade of alarms in an array of detectors called IceCube.
Within seconds IceCube had alerted an armada of astronomical satellites, including the Fermi Gamma-ray Space Telescope. That spacecraft traced the neutrino back to an obscure dot in the sky, a distant galaxy known as TXS 0506+056, just off the left shoulder of the constellation Orion, which was having a high-energy outburst of X-rays and gamma-rays.
While astronomers around the world scrambled to their telescopes to get in on the fun, the IceCube scientists scoured their previous data and found that there had been previous outbursts of neutrinos from the galaxy, which they nicknamed the “Texas source”, including an enormous neutrino outburst in 2014 and 2015.
Astronomers said the discovery could provide a long sought clue to one of the enduring mysteries of physics and the cosmos. Where does the rain of high-energy particles from space known as cosmic rays come from?
The leading suspects have long been quasars. They are supermassive black holes in the centres of galaxies where matter and energy get squeezed like toothpaste out of the top and bottom of a doughnut of doomed swirling material in a violent jet.
Now they know at least one in which that seems to be the case. TXS 0506+056 is a type of quasar known as a blazar, in which our line of sight from earth is along the jet — right down the gun barrel. The term blazar comes partly from BL Lacertae, a star-like object that turned out to be the first of these objects ever recognised.
“We have found the first source of cosmic rays,” said Francis Halzen, of the University of Wisconsin, Madison, and IceCube’s director, in an interview. Where exactly in the active galaxy, the neutrinos are produced will be a matter of debate,” he added in an email. “It is clear that the supermassive black hole provides the accelerator power,” he said, but how is a mystery.
The discovery is being announced in a series of papers by an international array of physicists and astronomers in Science and the Astrophysical Journal, and in a news conference sponsored by the National Science Foundation, which funds the IceCube Neutrino Observatory at the Amundsen-Scott South Pole Station.
Neutrinos are among the most plentiful particles in the universe — far outnumbering the protons and electrons out of which we are composed. They have no electrical charge and so little mass that it has not been accurately measured yet. They interact with other matter only by gravity and the so-called weak nuclear force and thus flow through us, earth and even miles of lead like ghosts.
Yet, in theory they are all over. Produced by radioactive decays of other particles, they are flooding us from nuclear reactions in the sun, distant supernova explosions and even the Big Bang. The previous great moment in neutrino astronomy happened in 1987, when some 25 neutrinos were recorded in three detectors on earth coincident with a supernova explosion in the Large Magellanic Cloud, a nearby galaxy.
The lure of neutrinos for astronomy is that it is possible to trace them back to their origins. Not only do they fly long distances and from otherwise impenetrable spots like the cores of stars at virtually the speed of light, but, by not having an electrical charge they are not affected by interstellar and intergalactic magnetic fields and other influences that scramble the paths of other types of cosmic particles, like protons and electrons. Neutrinos go as straight through the universe as Einsteinian gravity will allow.
IceCube, an international observatory run by 300 scientists from 12 countries, consists of more than 5,000 sensitive photomultiplier tubes embedded in grid encompassing one cubic kilometre of ice at the South Pole. When a neutrino very, very, very, very, very rarely hits an atomic nucleus in the ice, it produces a cone of blue light called Cerenkov radiation that spreads through the ice and is picked up by the photomultipliers.
IceCube was built, Halzen said, to find the source of cosmic rays, and the observatory has been recording neutrinos ever since it started working in 2011, but had not been able to pinpoint the sources of any of them until now. One reason, he said, was that the scientists had assumed the sources would be nearby, perhaps even in our own Milky Way galaxy.
But, TXS 0506+056, the Texas source, is very far away, some four billion light-years. It is one of the brightest objects in the universe, said Halzen.
The neutrino that set off the alarm in 2017 had an energy of some 300 trillion electron volts, by the units of energy and mass that physicists prefer. Which means it had been produced by a proton that had been a booster to that energy, nearly 50 times the energy delivered by the Large Hadron Collider at CERN, the biggest particle accelerator on earth.
Call it the Large Hadron Collider in the sky. Presumably, it is some kind of super-massive black hole rumbling in the heart of that distant galaxy.
For now, how this cosmic accelerator works in detail is a mystery.
Neutrinos are among the most plentiful particles in the universe — far outnumbering the protons and electrons out of which we are composed. They have no electrical charge and so little mass that it has not been accurately measured yet.