An international team of scientists has detected pulsed gamma-ray emissions from the Crab pulsar at far higher energies than expected.
The highly energized gamma rays are coming from an extreme object at the Crab Nebula's center known as a pulsar, the remains of the original star's core that collapsed in on itself into a super-dense, spinning neutron star.
Researchers found gamma rays with energies exceeding 100 billion electron- volts, higher energy levels than the current theoretical models can explain, from the fast spinning Crab Pulsar supernova that was discovered in 1968.
If you asked theorists a year ago whether we would see gamma-ray pulses this energetic, almost all of them would have said, 'No.' There's just no theory that can account for what we've found, said corresponding author Martin Schroedter of the Harvard-Smithsonian Center for Astrophysics (CfA).
The emissions were detected by the Very Energetic Radiation Imaging Telescope Array System of four 12-meter Cherenkov telescopes in Arizona. VERITAS, which 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, Santa Cruz, and the findings are published in Friday's issue of the journal Science.
Otte said that some researchers had told him that he was crazy to even look for pulsar emission in this energy realm.
Prior to these results, a phenomenon known as curvature radiation - where high-energy charged particles move along a curved magnetic field - was the leading explanation for the Crab's pulsed gamma-ray emissions. But this mechanism cannot account for gamma rays with energies above 100 GeV.
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. So it came as a real surprise when we found pulsed gamma-ray emission at energies above 100 GeV, said coauthor David Williams, adjunct professor of physics at UC Santa Cruz and a member of the VERITAS collaboration.
The Crab Nebula, which is some 6,500 light-years from Earth, was formed when a massive star exploded in a supernova that was observed on Earth in 1054.
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.
It turns out that being persistent and stubborn helps. These results put new constraints on the mechanism for how the gamma-ray emission is generated, Otte said.
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 can be better understood.
These are much, much higher energies than had been previously thought can come from a pulsar, Otte said. There was 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.