X-ray: NASA / CXC / ASU / J. Hester et al.; Optical: NASA / HST / ASU / J. Hester et al.; Radio: NRAO / AUI / NSF
This image of the Crab Nebula combines visible light (green) and radio waves (red) emitted by the remnants of a cataclysmic supernova explosion in the year 1054, and the X-ray nebula (blue) created inside the optical nebula by a neutron star (the collapsed core of the massive star destroyed in the explosion).
updated 10/6/2011 5:55:37 PM ET 2011-10-06T21:55:37

When astronomers detected intense radiation pumping out of the Crab Nebula, one of the most studied objects in space, at higher energies than anyone thought possible, they were nothing short of stunned.

The inexplicably powerful gamma-rays came from the very heart of the Crab Nebula, where an extreme object called a pulsar resides.

"It was totally not expected — it was absolutely jaw-dropping," Andrew McCann, a Ph.D. candidate at McGill University in Montreal, Canada, and a co-author of the new study, told Space.com. "This is one of the hottest targets in the sky, so people have been looking at the Crab Nebula for a long time. Now there's a twist in the tale. High-energy rays coming from the nebula are well-known, but coming from the pulsar is something nobody expected."

Details of the study, which was led by Nepomuk Otte, a postdoctoral researcher at the University of California, Santa Cruz, are published in the Oct. 7 issue of the journal Science.

The Crab's mysteries
The photogenic Crab Nebula is really the wreckage of a long-dead star that emitted an explosion of light that reached Earth in the year 1054, and was seen and recorded by Chinese and Native American skygazers. The dying star was located 6,500 light-years away from Earth in the constellation Taurus when it erupted in a brilliant supernova explosion.

At the heart of the nebula's colorful layers of gas is a so-called pulsar, which is the remains of the original star's core that collapsed in on itself into a super-dense, spinning neutron star. The Crab pulsar spins 30 times a second and is so dense that it has a greater mass than the sun.

A pulsar emits a continuous beam of radiation that sweeps around like a lighthouse, but appears to pulse when it is viewed through ground-based telescopes.

The gamma-ray beams that were detected from the Crab pulsar exceeded 100 billion electron-volts, stronger than anyone or any theories projected — a million times more energetic than medical X-rays and 100 billion times stronger than visible light, the researchers said.

"If you asked theorists a year ago whether we would see gamma-ray pulses this energetic, almost all of them would have said, 'No,'" study co-author Martin Schroedter, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., said in a statement. "There's just no theory that can account for what we've found."

NASA / ESA and Jeff Hester (Arizona State University)
The Hubble Space Telescope has caught the most detailed view of the Crab Nebula in one of the largest images ever assembed by the space-based observatory.

In fact, the findings were so unlikely that some researchers told Otte that he was crazy to look for such highly energetic emissions from the pulsar.

"It turns out that being persistent and stubborn helps," Otte said in a statement.

Shaking up the field
These new details of the Crab pulsar could change scientists' understanding of gamma-ray emissions and how they are generated, Otte added.

"We thought we understood the gamma-ray emission, and this was really becoming a foundational feature of our models, but that's now thrown out," McCann explained. "The reason why this is so exciting is that it's turning things around in the field."

The gamma-ray beams from the Crab pulsar were detected by the Very Energetic Radiation Imaging Telescope Array System (VERITAS), which is located at the Smithsonian's Whipple Observatory, just south of Tucson, Ariz.

Possible explanations for the Crab pulsar's intense beams have been suggested, but the researchers said that plenty more data will need to be collected before the mechanisms behind these gamma-ray pulses can be better understood.

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Scientists also expect to refine their observations once VERITAS undergoes an upgrade in the summer of 2012 to make its instruments more sensitive, McCann said. Other planned next-generation observatories — such as the Cherenkov Telescope Array (CTA), a project to build a very high-energy gamma-ray instrument — should also further this research.

"It's much more long term, but once CTA comes on, it's really going to write the book on this," McCann said.

VERITAS, which began collecting full-scale observations in 2007, is used to examine the remains of exploded stars, distant galaxies, powerful gamma-ray bursts, and to search for evidence of mysterious dark matter particles.


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