If astronomers are right, a ghostly particle that lit up an instrumented swath of ice beneath the South Pole on 22 September last year was a messenger from a distant galaxy. The particle was a neutrino, electrically neutral and almost massless, which means its path could be traced back to the extragalactic event that created it. Cued by IceCube, the Antarctic detector, the orbiting Fermi Gammaray Space Telescope found that the neutrino likely came from a far off blazar, a hugely bright source of radiation powered by a supermassive black hole.
Astronomers have long been tantalized by the prospect of using neutrinos, which move at almost the speed of light and rarely interact with other matter, to learn about violent cosmic events. The new finding, reported today in Science, could mark the founding event of neutrino astronomy. The detection also triggered a powerful example of another new trend, multimessenger astronomy, in which telescopes and other instruments studied the flaring blazar in all parts of the electromagnetic spectrum, from gamma rays to radio waves.
A neutrino-producing blazar could also help solve a decades-old mystery in astronomy: Where do the extremely high energy protons and other nuclei that occasionally bombard Earth come from? Known as ultrahigh-energy cosmic rays, these particles have a million times more energy than has ever been produced in an earthbound particle accelerator, but what boosts them to such colossal energies is unknown. Suspects have included neutron stars, gamma ray bursts, hypernovae, and the radiation-spewing black holes at the center of some galaxies, but whatever the source, high energy neutrinos are a likely byproduct. If the IceCube team is right, blazars could be the first confirmed source of these cosmic rays.
Researchers note, however, that the link between the neutrino and the blazar isn't rock solid. "It's a very mouthwatering observation and I very much hope it will be confirmed," says Pierre Sokolsky of the University of Utah in Salt Lake City. "If their interpretation of those observations is correct it will be revolutionary, extraordinary," says Eli Waxman of the Weizmann Institute of Science in Rehovot, Israel. But, he adds, "an extraordinary result needs extraordinary support, and the support is not quite extraordinary yet."
Completed in 2010, the IceCube neutrino detector snares these elusive particles in a cubic kilometer of Antarctic ice. When a neutrino hits a nucleus in the frozen water molecules, other particles fly off in recoil; as they decelerate, they emit light called Cherenkov radiation, which some of IceCube's 5160 light detectors may pick up. Based on the location, timing, and brightness of the detected light, researchers can reconstruct the path and energy of the neutrino.
Most of the neutrinos detected by IceCube originated nearby, spawned by cosmic rays hitting Earth's upper atmosphere. IceCube researchers eliminate those using a variety of methods, leaving the very few, very high energy neutrinos, above 30 trillion electron volts (TeV). In 2013, the IceCube team first revealed a handful of such events, arguing that their high energies and other properties showed they must have come from outside of our galaxy. The detector continues to bag about a dozen high energy neutrinos a year; when it gets a clean track with a well-defined direction, other telescopes scramble to see if there is an obvious cosmic source—until now, without success.
In 2016 IceCube's operators set up an alert service, with the hope of getting more telescopes at different wavelengths involved in the hunt. Then, last September, IceCube got lucky. A detected neutrino, dubbed IceCube-170922A and calculated to have an energy of 290 TeV, offered a relatively clear track back into space. An automatic alert went out less than a minute later.
Several observatories initially didn't see anything unusual. Six days later, the Fermi team reported the satellite had found that a blazar, known as TXS 0506+056 and just 0.1° away from the neutrino track suggested by IceCube, was especially bright, having started flaring a few months earlier. Soon, more than a dozen telescopes had studied the blazar. Blazars, like quasars, are distant cosmic beacons powered by supermassive black holes, which generate intense radiation and fire jets of particles from their poles. Blazars are exceptionally bright, astronomers believe, because their jets happen to be aimed straight at Earth.
IceCube and the other observers estimate the probability that the neutrino path and the blazar coincided by chance is roughly one in 740. Physicists and astronomers, however, aren't usually convinced that two phenomena are connected until there's no more than a one in 3.5 million, or 5 sigma, probability of a coincidence.
IceCube researchers also went back through almost a decade of data to see whether an excess of high-energy neutrinos had streamed from the same location before. They found a period of 150 days in late 2014 and early 2015 when IceCube detected around 13 more neutrinos than normal from that spot. It's not yet clear whether TXS 0506+056 was flaring at that time, but "the archival event was much more interesting" than the recent detection, says IceCube Principal Investigator Francis Halzen of the University of Wisconsin in Madison.
Sokolsky and Waxman agree that IceCube's September 2017 detection should strengthen the project's longstanding bid to massively increase the size of the instrument, which would also increase how many neutrinos it can detect and improve its pointing accuracy. Since IceCube was built, the team has found the ice is clearer than previously thought, so they believe they can make IceCube 10 times bigger while only doubling the number of light detectors, matching the $280 million cost of its original construction. The team is about to start experiments to test that. "With a 10-times-bigger detector, the answer [to where high energy neutrons come from] would be clear and obvious," Waxman says.