“Beyond Fermi” — Crab Nebula’s Neutron Star May Solve the Mystery of Cosmic Rays




The Crab Nebula, the remnant of a supernova explosion that was observed by Chinese and other astronomers in the year 1054, and one of the best-studied objects in the history of astronomy, may solve the mystery of cosmic rays. The nebula emits radiation across the entire electromagnetic spectrum, from gamma rays, ultraviolet and visible light, to infrared and radio waves. Most of what we see comes from very energetic particles (electrons), and astrophysicists can construct detailed models to try to reproduce the radiation that these particles emit. The currently accepted model was created by the Italian physicist Enrico Fermi in 1949. It now appears that he was only partially right.

The nebula is 6,500 light-years from Earth. At its center is a super-dense neutron star, rotating once every 33 milliseconds, shooting out rotating lighthouse-like beams of radio waves and light — a pulsar (the bright dot at image center). The nebula's intricate shape is caused by a complex interplay of the pulsar, a fast-moving wind of particles coming from the pulsar, and material originally ejected by the supernova explosion and by the star itself before the explosion.


The Hubble image above shows the very heart of the Crab Nebula including the mysterious central neutron star — it is the rightmost of the two bright stars near the center-right of this image. The neutron star is made entirely of neutrons (hence the name), it has the same mass as the Sun. Yet all of that mass is compressed into a sphere only a few tens of kilometers across. A neutron star is so dense that single teaspoon of its matter would weigh as much as a mountain.

A new study, by Federico Fraschetti at the University of Arizona, USA, and Martin Pohl at the University of Potsdam, Germany, reveals that the electromagnetic radiation streaming from the Crab Nebula may originate in a different way than scientists have traditionally thought: The entire zoo of radiation can potentially be unified and arise from a single population of electrons, a hypothesis previously deemed impossible.

The video below starts with a composite image of the Crab Nebula that was assembled by combining data from five telescopes spanning nearly the entire breadth of the electromagnetic spectrum: the Very Large Array, the Spitzer Space Telescope, the Hubble Space Telescope, the XMM-Newton Observatory, and the Chandra X-ray Observatory.

The video dissolves to the red-colored radio-light view that shows how a neutron star’s fierce “wind” of charged particles from the central neutron star energized the nebula, causing it to emit the radio waves. The yellow-colored infrared image includes the glow of dust particles absorbing ultraviolet and visible light. The green-colored Hubble visible-light image offers a very sharp view of hot filamentary structures that permeate this nebula. The blue-colored ultraviolet image and the purple-colored X-ray image shows the effect of an energetic cloud of electrons driven by the rapidly rotating neutron star at the center of the nebula.


According to the generally accepted model, once the particles reach a shock boundary, they bounce back and forth many times due to the magnetic turbulence. During this process they gain energy—in a similar way to a tennis ball being bounced between two rackets that are steadily moving nearer to each other—and are pushed closer and closer to the speed of light. Such a model follows an idea introduced by the Italian physicist Enrico Fermi in 1949.

"The current models do not include what happens when the particles reach their highest energy," said Federico, a staff scientist at the University of Arizona's Departments of Planetary Sciences, Astronomy and Physics. "Only if we include a different process of acceleration, in which the number of higher energy particles decreases faster than at lower energy, can we explain the entire electromagnetic spectrum we see. This tells us that while the shock wave is the source of the acceleration of the particles, the mechanisms must be different."

"The new result represents an important advance for our understanding of particle acceleration in cosmic objects, and helps to decipher the origin of the energetic particles that are found almost everywhere in the universe," adds co-author Martin Pohl.

The authors conclude that a better understanding is needed of how particles are accelerated in cosmic sources, and how the acceleration works when the energy of the particles becomes very large. Several NASA missions, including ACE, STEREO and WIND, are dedicated to studying the similar properties of shocks caused by plasma explosions on the surface of the sun as they travel to Earth, and so may add vital insights into these effects in the near future.

The Daily Galaxy via University of Arizona

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