In 1963, in the thick of the Cold War, the United States Air Force launched into space the first of a series of a dozen satellites designed to search for telltale signs of nuclear explosions. The United States, the United Kingdom and the U.S.S.R. had just signed a treaty prohibiting tests of nuclear weapons in the atmosphere, in outer space or underwater. The satellites, called “Vela” – short for “velador” or “watchman” in Spanish – were equipped with X-ray, neutron and gamma-ray detectors to keep an eye on how well the Soviet Union kept their word.
In 1967, one of the Vela satellites encountered a brilliant flash of gamma radiation. U.S. monitors went on the alert, keen to determine whether someone had broken the treaty. They discovered an eerie truth instead: Those high-energy flashes were happening on a regular basis, often once or twice a day. They were coming from all directions. And they were not coming from the Earth.
The satellites had discovered gamma-ray bursts, cataclysmic events about 100 times more powerful than a supernova that send jets of charged particles bolting through space at close to the speed of light. The particles radiate and accelerate gamma rays before shooting them into space.
Scientists still don't fully understand how gamma-ray bursts work, and recent results from a neutrino experiment located at the South Pole may only deepen the mystery.
Gamma rays were not the first curious particles scientists caught streaming from space. In 1912, physicist Victor Hess discovered that charged particles, most of them protons, constantly pelt Earth’s atmosphere in the form of cosmic rays. These projectiles collide with the atmosphere, setting off showers of invisible, short-lived particles.
It’s difficult to trace a cosmic ray back to its origins. Space’s magnetic field traps charged particles and swirls them around the galaxy like dandelion seeds in the wind for hundreds of thousands of years.
But scientists have a few theories about these wandering particles’ roots. Solar flares spit out the lowest-energy cosmic rays, those measured at about 1-10 gigaelectronvolts. Calamitous events within our galaxy, such as the collapse of dying stars, most likely produce somewhat higher-energy cosmic rays.
For half of a century, those were the only cosmic rays anyone knew about. But in 1962, scientists at the Volcano Ranch experiment in New Mexico stumbled upon a shocking anomaly: a cosmic ray with an energy above 1 billion GeV. Even more surprising, in early 1991, scientists at the Fly’s Eye experiment in Utah detected what some refer to as the “Oh-My-God” particle, a cosmic ray with an energy of 300 billion GeV.
Whoppers like these are rare. Just one or two might hit the same square mile of the planet in 100 years. It has been a complete mystery where they came from.
One thing scientists did know: When you accelerate a large number of protons together, they collide and produce rarely interacting particles called neutrinos. Laboratories that produce neutrino beams for experiments make them using beams of protons. Whatever was accelerating these cosmic ray particles must have also been a high-power neutrino source.
Gazing at the stars
Enter the IceCube experiment.
The IceCube experiment watches neutrinos a bit like people watch stars. To see a star is to see the particles of light that star has emitted. The particles’ travel time lets stars linger in our view some time after they have burned out.
Stars look like points in the sky because particles of light, called photons, travel in a straight line. They have no charge; unlike cosmic rays, they aren’t affected by our galaxy’s magnetic fields. Neither are neutrinos.
The physicists on the IceCube experiment built a neutrino detector using an array of photosensors to pinpoint the source of ultrahigh-energy cosmic rays. Whatever violent event accelerated these protons must have created an angry swarm of neutrinos as well.
According to theory, the particles that become cosmic rays pick up their energy from shockwaves produced in cataclysmic events such as the collapse of stars in space. Magnetic fields pull the particles through the same waves multiple times until they gain enough oomph to break free of the cycle.
Studying the math of this, physicists have found that, out of all known cosmic events, only two could possibly create ultrahigh-energy cosmic rays.
One is supermassive black holes continuously spitting jets of particles as they devour neighboring galaxies. This theory, though it only barely fits with the equation, has had its moment in the sun. Scientists studying cosmic rays at the Pierre Auger observatory reported in 2007 that they had found encouraging hints these types of supermassive black holes were the cause of ultrahigh-energy cosmic rays. But further data made the result look a lot less likely.
Only one other primary suspect remained: the mysterious, short-lived gamma-ray burst.
The big reveal
To build the IceCube experiment, scientists strung about 5,160 10-inch sensors together like Christmas tree lights, 60 sensors per strand, and sank them deep into a square kilometer of Antarctic ice. They lined the surface of the ice with about 320 additional sensors. The array of photomultiplier tubes detects a small fraction of the airy neutrinos that stream through the Earth.
Scientists began recording neutrino interactions in 2005, when they installed the first string of detectors. They continued to collect data, taking note when gamma-ray bursts occurred, until the detector was half-complete in 2010.
IceCube scientists used a blind analysis, which means that, in looking at their data, they first selected periods without gamma-ray bursts. They took into account how many neutrino events they saw unrelated to the big events.
They calculated that, once they tuned out this background noise, they would see an average of 0.04 neutrinos per gamma-ray burst. During their years of data-taking, the sensors had lived through 220 gamma-ray bursts. That meant that, if the physicists’ calculations were correct, and if the gamma-ray bursts were the source of ultrahigh-energy cosmic rays, they would see a total of eight or nine neutrinos events once the blinders came off.
The day arrived to finally reveal whether IceCube had triumphed in linking two of the most curious concepts in cosmology. The physicists unblinded their data.
They had detected nothing. As physicist Sebastian Boser reported at this week’s Rencontres de Moriond meeting, IceCube had recorded 0 neutrino events above background levels during the gamma-ray bursts.
The mystery remains. The IceCube array, now completed, will continue listening for calling neutrinos for another year before the scientists unblind their data again. Physicists' two candidates for the source of ultrahigh-energy cosmic rays may soon be off the table.
It could turn out that the other-worldly process that fires high-energy cosmic-ray bullets into our galaxy is something no one has ever seen before. Who knows what we’ll stumble upon in the future, perhaps while keeping watch for something entirely different?