August 29, 2008

Researchers Gain New Knowledge From Crab Nebula

Researchers say a strong particle accelerator has been located in the Crab Nebula, a magnetic field surrounding the stellar corpse at the nebula's center.

The discovery was found with a tricky measurement that showed polarized high-energy radiation near the star.  It's electrical field aligned neatly with the star's axis.

The Crab Nebula is the remaining bits of a supernova that occurred in 1054 CE.  The subsequent pulsar left behind is a dense corpse.

The pulsar emits particles and electromagnetic fields as it spins around 30 times per second.

Many of the particles emit high-energy radiation in the form of gamma and x-rays when they are accelerated by magnetic fields.  Scientists have been unable to determine why the acceleration is happening.

Researchers in the UK, led by Tony Dean of the University of Southampton, believe the acceleration is happening close to the pulsar.

They base their decision on data obtained by Europe's INTEGRAL satellite which shows that 46% of the pulsar's gamma-ray emission was polarized, meaning the photons' electromagnetic fields aligned in a common orientation.

"That's very high for anything in astrophysics," says David Thompson of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Such a high percentage of polarization means you really have to have very good conditions "“ a magnetic field that is very ordered."

A well-ordered magnetic field is believed to appear close to a pulsar that has a magnetic field a trillion times as strong as Earths.

The pulsar's magnetic field is similar to a bar magnet.  It's magnetic field lines appear from one pole and bend around before returning to the other pole.

"But once you get away from the pulsar, then you get a much more complex situation because the field begins to break up into little patches and knots," Thompson told.

The new data proposes that the particles are being accelerated near the pulsar before the electromagnetic field can become twisted.

According to Thompson, the data fits in with predictions that are difficult to prove observationally.  A very small number of polarization measurements have been done at gamma-ray and x-ray wavelengths.

Determining the polarization direction of photons is nearly impossible with current instruments, especially at great distances.

"It'll be hard to reproduce this in another pulsar," agrees Thompson. That's because the Crab Nebula is only 6500 light years away, making it a relatively close object for research.

"The Crab is everybody's favorite astrophysical lab," he says. "It's so well studied that the easy questions have been answered. Now we're down to the more complicated issues."

The new information about the magnetic field accelerating particles will allow scientists to infer the power of more distant objects.

"Pulsars and their surrounding regions are examples of physics under extreme conditions. Anything you can do to learn about how they work helps us understand the basic physics of particle acceleration and magnetic field generation," said Thompson.


Image Caption: This image shows the direction of polarisation (alignement) of the high-energy radiation emitted by the Crab Nebula, as detected by ESA's Integral gamma-ray observatory. The shaded part represents the error in the determination of this direction. This direction is remarkably aligned with the inner jets of the Crab. On their turn, these are aligned with the rotation axis of the pulsar located at the centre of the system.

The Crab Nebula image in the background was obtained by combining an optical image by NASA/ESA's Hubble Space Telescope and an X-ray image by NASA's Chandra X-ray observatory.

Credits: NASA/CXC/ASU/J. Hester et al.(for the Chandra image); NASA/HST/ASU/J. Hester et al. (for the Hubble image)


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