Researchers suggest high energy emissions from Crab Nebula come from wind

Feb 16, 2012 by Bob Yirka report
This is a mosaic image, one of the largest ever taken by NASA's Hubble Space Telescope of the Crab Nebula, a six-light-year-wide expanding remnant of a star's supernova explosion. Image: NASA

(PhysOrg.com) -- An international team of physicists studying the Crab Nebula have offered a new theory to explain its extraordinarily high energy emissions that have intrigued space scientists for years. The team, led by Felix Aharonian of the Dublin Institute for Advanced Studies, suggests that instead of the energy being emitted by the pulsar that sits at the center of the nebula, it comes instead from wind, as they describe in their paper in Nature, generated by the pulsar.

The , of special interest to scientists for years, is what remains after a exploded close to a thousand years ago, and the pulsar is what was left over; a very dense neutron star that spins at thirty times per second. Because the pulsar appears to pulse, hence it’s name, scientists have assumed that the emissions from the nebula came directly from it, especially since the energy comes in bursts that come at the same rate as the pulses. But recent research has shown that what appears to be a pulse from the pulsar is actually more than that. The pulsar actually generates a continues beam of radiation that appears to pulse only when that beam heads our way.

In this new research Aharonian and his team traced back the energy emissions from the nebula and found that it didn’t lead straight to the pulsar, but to a point somewhat near to it. To explain this, they’ve come up with a theory that suggests that the energy from the pulsar moves away from the pulsar into the rest of the nebula, where it is captured by a that carries it out into space. Though this wind cannot be seen, its presence can be theorized based on the behavior of nebula.

In studying the behavior of the wind, the team has found that in its initial stages, it appears to be dominated by electromagnetic energy, but then becomes much more kinetic at some point by some process that is still not understood. They also believe they can estimate where it originates by noting that if it were too close to the , they gamma ray emissions would have to be higher. Conversely, if it were any farther than a certain point, the photons it carries would be too dim to be seen.
Aharonian’s group next plans to focus on explaining why the wind picks up speed as it moves out of the nebula.

Explore further: Spectacular supernova's mysteries revealed

More information: Abrupt acceleration of a ‘cold’ ultrarelativistic wind from the Crab pulsar, Nature (2012) doi:10.1038/nature10793

Abstract
Pulsars are thought to eject electron–positron winds that energize the surrounding environment, with the formation of a pulsar wind nebula. The pulsar wind originates close to the light cylinder, the surface at which the pulsar co-rotation velocity equals the speed of light, and carries away much of the rotational energy lost by the pulsar. Initially the wind is dominated by electromagnetic energy (Poynting flux) but later this is converted to the kinetic energy of bulk motion. It is unclear exactly where this takes place and to what speed the wind is accelerated. Although some preferred models imply a gradual acceleration over the entire distance from the magnetosphere to the point at which the wind terminates, a rapid acceleration close to the light cylinder cannot be excluded. Here we report that the recent observations of pulsed, very high-energy γ-ray emission from the Crab pulsar are explained by the presence of a cold (in the sense of the low energy of the electrons in the frame of the moving plasma) ultrarelativistic wind dominated by kinetic energy. The conversion of the Poynting flux to kinetic energy should take place abruptly in the narrow cylindrical zone of radius between 20 and 50 light-cylinder radii centred on the axis of rotation of the pulsar, and should accelerate the wind to a Lorentz factor of (0.5–1.0) × 106. Although the ultrarelativistic nature of the wind does support the general model of pulsars, the requirement of the very high acceleration of the wind in a narrow zone not far from the light cylinder challenges current models.

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User comments : 6

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Tuxford
1 / 5 (6) Feb 16, 2012
LaViolette superwave theory accounts for an external source of illumination for the Crab Nebula. Emitted from our galactic core, the superwave of cosmic ray electrons that triggered our last ice age is now passing through the Crab face-on, accounting for the expanding ring features observed thereon. The Crab is near the galactic anti-center with respect to Earth, permitting us to see the resulting synchrotron emissions therefrom, as the the superwave passes through the Crab and interacts with various magnetic fields therein.

http://www.physor...dog.html

This theory also includes high energy synchrotron radiation emitted from our galactic core, an unexpected feature that is now acknowledged.

http://www.physor...int.html

Tuxford
1 / 5 (3) Mar 01, 2012
LaViolette gives more detailed explanations for superwave illumination of the Crab Nebula below:

http://starburstf...mment-31

He comments on the source of the flaring activity being caused by the cosmic ray electrons interacting with the magnetic fields within the nebula, producing synchrotron radiation.

http://starburstf...mment-30

And the expanding ring structure explanation is detailed here:

http://starburstf...mment-29

And why recent flares are too energetic to be caused by the pulsar:

http://starburstf...g/?p=182
Tuxford
1 / 5 (3) Mar 01, 2012
LaViolette points out that the pulsar is slight off-center in the nebula from our perspective. As the Crab is located near the galactic anti-center, the impact of the superwave would be centered slightly offset from the pulsar. The resulting expanding rings would therefore be slightly offset, as observed by these researchers. However, they propose some seemingly complex wind theory to account for this displacement.

Again, why not consider a simpler explanation, like superwave illumination?
CardacianNeverid
3.4 / 5 (5) Mar 02, 2012
Again, why not consider a simpler explanation, like superwave illumination? -StepfordSon

For the same reason any other crank notion isn't considered.
Tuxford
1 / 5 (2) Mar 04, 2012
LaViolette has now commented on this pulsar offset acknowledgement, pointing out that in his 1983 dissertation, this was a key point suggesting the source of the emissions was not the pulsar. He disagrees with this wind theory.

http://starburstf...g/?p=238
Vendicar_Decarian
5 / 5 (1) Mar 04, 2012
Impossible. Denialist Tards insist that wind energy can't power anything.