Hunt for cosmic ray source falls short

Experiment’s dearth of neutrinos may burst gamma-ray bubble

Instead of clearing up a half-century-old mystery, scientists have tossed a bit of mud into an already murky pool of suspects behind high-energy cosmic rays. New results from the IceCube Collaboration now cast doubt on gamma-ray bursts as ready producers of these enigmatic particles that strike the Earth with energies exceeding 10 billion-billion electron volts.

The IceCube Laboratory (shown) sits atop the South Pole, but buried beneath the ice is a massive neutrino-detecting telescope that fills a cubic kilometer of space. New experiments there did not detect neutrinos after gamma-ray bursts, thought to be a prime producer of energetic cosmic rays. S. Lidstrom/NSF

But there’s some wiggle room in the evidence, and if theorists rejigger equations describing the cosmic objects, gamma-ray bursts could still be in the lineup, scientists report April 19 in Nature.

The new work is based on results from the IceCube neutrino telescope, a cubic kilometer of detectors buried beneath the South Pole. Over a period of two years, the telescope didn’t detect any of the neutrinos expected to arrive following 307 gamma-ray bursts. Neutrinos act as proxies indicating that cosmic rays are produced.

“Either gamma-ray bursts cannot be the source of all ultra-high-energy cosmic rays, or there has to be some physics going on inside the gamma-ray burst that makes neutrino production different” from what scientists expected, says Abigail Vieregg, a physicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Ultra-high-energy cosmic rays, which are actually charged subatomic particles, arrive at Earth with millions of times the energy generated inside CERN’s Large Hadron Collider. Since first detecting these rays in 1962, scientists have sought the astrophysical accelerator whence the rays come — an object capable of super-flinging particles across the cosmos.

“You expect it to be something very violent,” says astrophysicist Nathan Whitehorn, a study coauthor at the University of Wisconsin-Madison. Gamma-ray bursts are prime suspects.

For two years, IceCube scientists waited expectantly for neutrinos to arrive after gamma-ray bursts. Researchers focused on finding the muon subtype, which leaves long footprints that point toward home. “If we measure the arrival direction on Earth, we can look back and see where they came from,” says astrophysicist and coauthor Spencer Klein of Lawrence Berkeley National Laboratory, in Berkeley, Calif.

But no neutrinos arrived.

“I’m kind of inclined to think that it’s because gamma-ray bursts are not the source of the ultra-high-energy cosmic rays in the universe,” Klein says.

Klein and Whitehorn point to active galactic nuclei as the next most likely cosmic accelerator. These blazing cores house spinning supermassive black holes, behemoths that feast on light and matter while belching energetic jets capable of firing cosmic rays at Earth.

Vieregg and others think it’s still possible that gamma-ray bursts are the culprit but say relevant theories will need to be re-worked. IceCube’s non-detection of neutrinos should help. “Measuring zero has been the most important contribution so far,” says astroparticle physicist Peter Gorham of the University of Hawaii at Manoa. “But we don’t want to keep doing that forever.”

If it’s not gamma-ray bursts, or active galactic nuclei, that are accelerating cosmic rays, then what? “I don’t know,” says astrophysicist James Buckley of Washington University in St. Louis. “I would choose the most mundane thing I could think of.”

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