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Crab Nebula’s Pulsar May Be Fast Particle Accelerator

Image: Crab Nebula gets the “Blues” by Danny Lacrue via HubbleSite

The Crab Nebula (also designated M1 or NGC 1952) is visible through small telescopes, which has allowed astronomers to observe its growth and evolution since the supernovae that created it became visible in 1054 CE. A pulsar was found in the center of the Crab in 1968.

This rapidly rotating neutron star is the core of the star that went supernova to make the nebula. In the intervening decades, x-ray, gamma ray, and radio observations have mapped the region of the nebula closest to the pulsar. During that mapping, it became apparent that the Crab pulsar is one of the brightest sources of gamma rays observable from Earth.

Despite all of those observations, we still don’t fully understand the Crab’s precise gamma ray spectrum, particularly recently observed pulses of intense gamma radiation seen by the Fermi Gamma-ray Space Telescope. Existing models certainly do well at describing much of the complex interplay between the intense magnetic fields of the pulsar and the winds of charged particles flowing outward. But no single scheme seems sufficient to cover all the observed phenomena.

A potentially promising new model, proposed by F. A. Aharonian, S. V. Bogovalov, and D. Khangulyan, may fill in some of these blanks. It proposes that areas near the pulsar are acting as rapid particle accelerators, but don’t boost electrons and heavier particles to the same extent.

Pulsars are exceedingly small despite their high mass: According to typical neutron star models, the Crab pulsar is approximately 30 kilometers in diameter, but contains nearly double the mass of our Sun. The intense gravitational influence and rapid rotation of pulsars place them firmly in the realm of relativity, while intense magnetic fields carry the enormous amounts of energy we typically encounter in particle accelerators.

In the region immediately surrounding the Crab pulsar, there is enough energy to produce pairs of electrons and positrons, which flow outward into the surrounding gas. This total flow is the pulsar wind, a plasma (an electrically neutral substance consisting of separate positive and negative charges) that moves very close to the speed of light.

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