The Crab Nebula Reveals Its Secrets

Though it's only 10 miles across, the amount of energy the pulsar at its core releases is enormous, lighting up the Crab Nebula until it shines 75,000 times more brightly than the sun. The nebula, one of our best-known and most stable neighbors in the winter sky, is shocking scientists with a propensity for fireworks—gamma-ray flares set off by the most energetic particles ever traced to a specific astronomical object. The discovery, reported today by scientists working with two orbiting telescopes, is leading researchers to rethink their ideas of how cosmic particles are accelerated.

"We were dumbfounded," said Roger Blandford, who directs the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University. "It's an emblematic object," he said; also known as M1, the Crab Nebula was the first astronomical object catalogued in 1771 by Charles Messier. "It's a big deal historically, and we're making an amazing discovery about it."

Blandford was part of a KIPAC team led by scientists Rolf Buehler and Stefan Funk that used observations from the Large Area Telescope, one of two primary instruments aboard NASA's Fermi Gamma-ray Space Telescope, to confirm one flare and discover another.

The Crab Nebula, and the rapidly spinning neutron star that powers it, are the remnants of a supernova explosion documented by Chinese and Middle Eastern astronomers in 1054. After shedding much of its outer gases and dust, the dying star collapsed into a pulsar, a super-dense, rapidly spinning ball of neutrons that emits a pulse of radiation every 33 milliseconds, like clockwork.

Most of the Crab Nebula's  energy is contained in a particle wind of energetic electrons and positrons traveling close to the speed of light. These electrons and positrons interact with magnetic fields and low-energy photons to produce the famous glowing tendrils of dust and gas Messier mistook for a comet over 300 years ago.

The particles are even forceful enough to produce the gamma rays the LAT normally observes during its regular surveys of the sky. But those particles did not cause the dramatic flares.

Each of the two flares the LAT observed lasted mere days before the Crab Nebula's gamma-ray output returned to more normal levels. According to Funk, the short duration of the flares points to synchrotron radiation, or radiation emitted by electrons accelerating in the magnetic field of the nebula, as the cause. The flares were caused by super-charged electrons of up to 10 peta-electron volts, or 10 trillion electron volts, 1,000 times more energetic than anything the world's most powerful man-made particle accelerator, the Large Hadron Collider in Europe, can produce, and more than 15 orders of magnitude more energetic than photons of visible light. 

 

"The strength of the gamma-ray flares shows us they were emitted by the highest-energy particles we can associate with any discrete astrophysical object," Funk said.

"The fact that the intensity is varying so rapidly means the acceleration has to happen extremely fast," added Buehler. This challenges current theories about the way cosmic particles are accelerated, which cannot easily account for the extreme energies of the electrons or the speed with which they're accelerated.

The KIPAC scientists need a closer look at higher resolutions and in a variety of wavelengths before they can make any definitive statements about the mechanisms behind the Crab Nebula's gamma-ray flares. "We thought we knew the essential ingredients of the Crab Nebula," Funk said, "but that's no longer true. It's still surprising us."

The Chandra images in the collage below were made over a span of several months (ordered left to right, except for the close-up). They provide a stunning view of the activity in the inner region around the Crab Nebula pulsar, the rapidly rotating neutron star seen as a bright white dot near the center of the images.

A wisp can be seen moving outward at half the speed of light from the upper right of the inner ring around the pulsar. The wisp appears to merge with a larger outer ring that is visible in both X-ray and optical images.

The inner X-ray ring consists of about two dozen knots that form, brighten and fade. As a high-speed wind of matter and antimatter particles from the pulsar plows into the surrounding nebula, it creates a shock wave and forms the inner ring. Energetic shocked particles move outward to brighten the outer ring and produce an extended X-ray glow.

Enormous electrical voltages generated by the rotating, highly magnetized neutron star accelerate particles outward along its equator to produce the pulsar wind. These pulsar voltages also produce the polar jets seen spewing X-ray emitting matter and antimatter particles perpendicular to the rings.

 

  

The ESO image above is an enlargement of a three color composite of the Crab Nebula, as observed with the FORS2 instrument in imaging mode in the morning of November 10, 1999. The blue light is predominantly emitted by very high-energy ("relativistic") electrons that spiral in a large-scale magnetic field (so-called synchrotron emission). It is believed that these electrons are continuously accelerated and ejected by the rapidly spinning neutron star at the center of the nebula.

Source: The Daily Galaxy via Science Express and the lSLAC National Accelerator Laboratory