Crab Pulsar Power Beams Baffle Scientists

A Crab Pulsar star has emitted the most highly energized gamma rays that have ever been observed from a pulsar system, raising questions among scientists over how these energetic pulses are formed.

Using the Very Energetic Imaging Telescope Array System (VERITAS), a group of international astrophysicists spotted gamma rays with energies exceeding 100 billion electron-volts emitted 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 highly energized gamma rays are emitted from the center of the pulsar, which is a spinning neutron star that is born from the collapsed core of a massive star. The pulsar's enormous magnetic field is known to gather up particles and accelerate them. Scientists believe that beams of radiation are generated from the crab pulsar's spinning magnetic fields that rotate at about 30 times a second.

The rays were observed by scientists for 107 hours over the course of three years using the powerful gamma ray observatory ,VERITAS, at the Whipple Observatory in Arizona. These rays are invisible to the human eye and the only way telescopes on earth can detect their path is by observing the path they take in the planet's atmosphere. The Cherenkov telescopes used at VERITAS detect the faint, extremely short flashes of blue light that these particles emit.

NASA's Fermi Gamma-ray Space Telescope was the first to suggest that the Crab Pulsar might emit more highly energized radiation that expected by scientists. Fermi only measures gamma rays up to energy of 20 gigaelectronvolts (GeV), but the data gathered suggest that there were more energetic particles than the telescope has caught, which led to the latest study of the pulsar.

Nepomuk Otte, a corresponding author of the study and postdoctoral researcher at the University of California, Santa Cruz, was told by some researchers that he was crazy to look for pulsar emission in this energy realm.

It turns out that being persistent and stubborn helps, Otte said. These results put new constraints on the mechanism for how the gamma-ray emission is generated.

The most recent observation has left scientists puzzled as to details of how the most energetic particles are formed and shown how elusive the complete understanding is of the process.

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, said David Williams, adjunct professor of physics at UC Santa Cruz and a member of the VERITAS collaboration.

Some possible scenarios to explain the data have been put forward, but it will take more data, or even a next-generation observatory, to really understand the mechanisms behind these gamma-ray pulses, researchers of the study reported.

Results of the study were published in the Oct.7 issue of Science. VERITAS continues the tradition of Whipple's 10-meter telescope. It is comprised of an array of four 12-meter-diameter Cherenkov telescopes. VERITAS began full-scale observations in September 2007. The telescopes are used to study the remnants of exploded stars, distant galaxies, powerful gamma-ray bursts, and to search for evidence of mysterious dark matter particles.