October 6, 2011
Researchers Detect High Energies Coming From Crab Pulsar
Astrophysicists have detected pulsed gamma-ray emissions from the Crab pulsar with energies that exceed 100 billion electron-volts (GeV).
These gamma-ray pulses surpass what current theoretical models of pulsars can explain. The pulses were detected by the VERITAS telescope array at the Whipple Observatory in Arizona.
Nepomuk Otte, a postdoctoral researcher at the University of California, Santa Cruz, said some researchers had told him he was crazy to even look for pulsar emission in this energy realm.
"It turns out that being persistent and stubborn helps," Otte said in a press release. "These results put new constraints on the mechanism for how the gamma-ray emission is generated."
The Crab pulsar is a rapidly spinning neutron star that exploded in a supernova in the year 1054 to leave behind the Crab Nebula.
The Nebula rotates at about 30 times a second and the pulsar has a co-rotating magnetic field from which it emits beams of radiation.
"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," coauthor David Williams, adjunct professor of physics at UC Santa Cruz and a member of the VERITAS collaboration, said in a press release.
Scientists thought that curvature radiation was the cause of the Crab's pulsed gamma-ray emission. However, Otte said this mechanism cannot account for gamma rays with energies above 100 GeV.
"The conventional wisdom was that the dominant mechanism is curvature radiation. But the VERITAS results have shown that there must be a different mechanism at work," Otte said in a press release. "Curvature radiation can explain the lower-energy emission, but we really don't know what causes the very high-energy emission."
It is still unclear whether one mechanism dominates at all gamma-ray energies, or if curvature radiation dominates at lower energies and something like inverse Compton dominates at higher energies.
The researchers reported their findings in the October 7 issue of Science.
Image 1: An artist's conception of the pulsar at the center of the Crab Nebula, with a Hubble Space Telescope photo of the nebula in the background. Researchers using the Veritas telescope array have discovered pulses of high-energy gamma rays coming from this object. Credit: David A. Aguilar / NASA / ESA
Image 2: This artist's conception shows the Crab Nebula pulsar, which astronomers discovered to be sending out pulses of gamma rays with energies exceeding 100 billion electron-volts (100 GeV). A pulsar is a spinning neutron star - the collapsed core of a massive star that exploded as a supernova. Credit: David A. Aguilar (CfA)
Image 3: An artist´s rendering of the VERITAS array detecting gamma-ray pulses from the Crab Nebula. Credit: JosÃ© Francisco Salgado based on images by M. SubbaRao, S. Criswell, B. Humensky, and J.F. Salgado
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