NASA
A Hubble visible light image of the Crab Nebula inset against a full-sky gamma ray map showing the location of the nebula (crosshairs).
updated 5/11/2011 7:00:06 PM ET 2011-05-11T23:00:06

The Crab Nebula, the dusty remains of an exploded star, has unleashed a surprisingly massive flare that is five times more powerful than any eruption previously seen from the celestial object, leaving scientists struggling to explain the event, NASA says.

The so-called "superflare" was detected on April 12 by NASA's Fermi Gamma-ray Space Telescope, which is continuously mapping the sky in gamma ray wavelengths in search of gamma-ray bursts, the brightest explosions in the universe.

The Crab Nebula's strong outburst lasted six days, and its exact cause has scientists scratching their heads, especially since the superflare followed an earlier gamma-ray flare from the nebula in January.

"These superflares are the most intense outbursts we've seen to date, and they are all extremely puzzling events," said Alice Harding at NASA's Goddard Space Flight Center in Greenbelt, Md., in a statement. "We think they are caused by sudden rearrangements of the magnetic field not far from the neutron star, but exactly where that's happening remains a mystery."

The outburst observed by Fermi was likely triggered by electrons with energies 100 times greater than can be achieved in any particle accelerator on Earth, scientists said. This makes them the highest-energy electrons known to be associated with any galactic source.

Based on the rise and fall of gamma rays during the April outbursts, scientists estimate that the size of the emitting region must be comparable to our entire solar system.

The Crab Nebula's legacy
The spectacular and colorful Crab Nebula is the wreckage of a dying star that emitted an explosion of light that reached Earth in the year 1054. The former 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 an expanding gas cloud lies what is left of the original star's core, a super-dense neutron star that spins 30 times a second. With each rotation, the star swings intense beams of radiation toward Earth, creating the pulsed emission characteristic of spinning neutron stars, which are also known as pulsars.

Apart from these pulses, astrophysicists thought the Crab Nebula was a virtually constant source of high-energy radiation. But, in January, scientists representing a variety of space-based observatories, including NASA's Fermi, Swift and Rossi X-ray Timing Explorer, reported long-term changes in brightness at X-ray energies.

"The Crab Nebula hosts high-energy variability that we're only now fully appreciating," said Rolf Buehler, a member of the Fermi Large Area Telescope (LAT) team at the Kavli Institute for Particle Astrophysics and Cosmology, a facility jointly located at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University in California.

Monitoring the Crab Nebula
Since 2009, Fermi and the Italian Space Agency's AGILE satellite have detected several short-lived gamma-ray flares at energies that are hundreds of times higher than the nebula's observed X-ray variations. The observed flares measured energies greater than 100 million electron volts. In perspective, visible light has energies between 2 and 3 electron volts.

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The superflare that was detected on April 12 by Fermi's LAT and later by the Italian AGILE satellite, grew about 30 times more energetic than the nebula's normal gamma-ray output, and was about five times more powerful than any previously seen outbursts. On April 16, an even brighter flare erupted, but within a couple of days, the unusual activity completely faded out.

The Crab Nebula's high-energy emissions are still mysterious, but are thought to be the result of physical processes that tap into the neutron star's rapid spin. Theorists generally agree that the flares must arise within about one-third of a light-year from the neutron star, but efforts to locate them more precisely have so far been unsuccessful.

Since September 2010, NASA's Chandra X-ray Observatory has routinely monitored the Crab Nebula in an effort to identify X-ray emissions associated with the outbursts. When Fermi scientists alerted astronomers to the onset of a new flare, Martin Weisskopf and Allyn Tennant at NASA's Marshall Space Flight Center in Huntsville, Ala., activated a set of preplanned observations using Chandra.

"Thanks to the Fermi alert, we were fortunate that our planned observations actually occurred when the flares were brightest in gamma rays," Weisskopf said. "Despite Chandra's excellent resolution, we detected no obvious changes in the X-ray structures in the nebula and surrounding the pulsar that could be clearly associated with the flare."

Scientists are still searching for the answer, but they think the flares occur as the intense magnetic field near the pulsar undergoes sudden restructuring. Such changes can accelerate particles like electrons to rapid velocities near the speed of light. As these high-speed electrons interact with the magnetic field, they emit powerful gamma rays.

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