Superenergetic Crab Pulsar Gamma Rays Puzzle Scientists

Astronomers have detected pulsed gamma rays from the neutron star at the heart of the Crab Nebula with energies far higher than their theoretical pulsar models can explain.

Researchers found gamma rays with energies exceeding 100 billion electronvolts -- higher energy levels than current theoretical models can explain -- from the fast-spinning Crab Pulsar supernova that was discovered in 1968.

The emissions were detected by the Very Energetic Radiation Imaging Telescope Array System of four 12-meter Cherenkov telescopes in Arizona. VERITAS, since it began collecting full-scale observations in 2007, is used to examine the remains of exploded stars, distant galaxies, and powerful gamma-ray bursts and to search for evidence of mysterious dark-matter particles.

The study team was led by Nepomuk Otte, a postdoctoral researcher at the University of California at Santa Cruz. The findings were published in Friday's issue of the journal Science.

After many years of observations and results from the Crab, we thought we had an understanding of how it worked, and the models predicted an exponential decay of the emission spectrum above around 10 GeV [gigaelectronvolts]. So it came as a real surprise when we found pulsed gamma-ray emission at energies above 100 GeV, said co-author David Williams, adjunct professor of physics at UC-Santa Cruz and a member of the VERITAS collaboration.

The Crab Nebula, some 6,500 light-years from Earth, was formed when a massive star exploded in a supernova that was observed on Earth in 1054. At the heart of the nebula's colorful layers of gas is a so-called pulsar, the remains of the original star's core that collapsed in on itself into a superdense, spinning neutron star.

It is most typical for pulsars to be ejected from the stellar wreckage during a supernova. But in the case of the Crab system, the pulsar remained at its center, producing radiation that covers the entire electromagnetic spectrum.

Possible explanations for the Crab Pulsar's intense beams have been suggested, but the researchers said much more data would need to be collected before the mechanisms behind these gamma-ray pulses could be better understood.

These are much, much higher energies than had been previously thought can come from a pulsar, Otte said. There is something missing in the models of the cosmic particle accelerators that give rise to the gamma rays, he added. It's a very radical change to the picture of how we believe gamma-ray emission comes from pulsars.