Crab Pulsar gamma rays baffle scientists

Posted by Kate Taylor

Scientists say the Crab Pulsar is blasting out gamma rays at a higher rate than can be explained by current scientific models.       

It's emitting the highest-energy gamma rays ever observed from a pulsar - a highly magnetized and rapidly spinning neutron star - at more than 100 billion electron volts. That's more than 50 billion times more energy than the visible light from the sun.

"This is the highest energy pulsar system ever detected," said Rene Ong, a UCLA professor of physics and astronomy and spokesperson for the VERITAS collaboration at the Harvard–Smithsonian Center for Astrophysics. "It is a completely new and surprising phenomenon for pulsars."

All previous observations of pulsars have indicated that the radiation cuts off at the high energies the team observed.

"It means the radiation we detect must be a new component that was completely unexpected," says Ong.

The Crab Pulsar spins at about 30 times a second, emitting gamma rays through 'curvature radiation', an effect that creates a lighthouse-like beacon that winks on and off.

When the gamma rays reach Earth, they smash into our atmosphere, causing what Ong describes as the electromagnetic equivalent of a sonic boom.
The collision creates a shower of visible light more than six miles above the ground that can be recorded by VERITAS.

"The atmosphere is an integral part of our measurement system, which makes VERITAS different from conventional telescopes," says Ong.

The Crab Nebula is one of the most widely studied astronomical objects in the northern hemisphere. It's around 6,500 light-years from Earth, and was formed when a massive star exploded in a supernova that was observed on Earth in the year 1054.

Ong describes the system as the 'Rosetta Stone of astronomy', because it's been observed at every conceivable wavelength of light.

"The Crab Pulsar is considered among the best understood systems in all of astronomy, yet here we have found something totally new," he says.

"It is astronomy in a completely new light; we are seeing phenomena that you just can't explore with optical light or X-rays, or even low-energy gamma rays."

The light detected by the VERITAS experiment can't be explained by the curvature radiation, and likely comes from regions well outside the high–magnetic field region close to the neutron star.

"The pulse duration of the radiation we see is almost three times shorter than that seen at other gamma ray energies," says Ong.

"This was very surprising, and means this new radiation is probably coming from a different physical region of the star's outer magnetosphere."

Ong hopes his research may shed some light on the mystery of cosmic rays.

"We are bombarded by high-energy particles from all over the cosmos that reach unimaginable energies," he says. "These cosmic rays are an important energy source in our galaxy, yet we have no clue where they are coming from."