“Superflares” in Crab Nebula
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A Hubble visible light image of the Crab Nebula inset against a full-sky gamma ray map showing the location of the nebula (croshairs). Credit: NASA
On April 12, NASA’s Fermi gamma-ray Space Telescope detected a flare in the Crab Nebula many times more powerful than any previously detected outburst. The Crab Nebula is the debris leftover from a supernova explosion that happened around 1054 C.E. Over 6,000 light years distant, at the heart of the Crab Nebula is a super dense Neutron Star. As with any Neutron star, the core of the Crab Nebula sends out intense “pulses” of radiation. If you want to look at the Crab Nebula in a telescope, it is located in the constellation Taurus.
Most researchers had thought the Crab Nebula, aside from its regular pulses was a nearly constant source of high-energy emission, however this past January, scientists working with data from several orbiting observatories, including NASA’s Fermi, Swift and Rossi craft, reported long-term brightness changes in the X-ray portion of the electromagnetic spectrum.
“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.
Since 2009, Fermi and the AGILE satellite have detected many short duration flares in the gamma ray portion of the spectrum, and at energies greater than 100 million electron volts (eV), which is hundreds of times higher than the energies of the nebula’s X-ray emissions. To compare, visible light has energies between 2 and 3 eV.
These images show the number of gamma rays with energies greater than 100 million electron volts from a region of the sky centered on the Crab Nebula. In both images, the bright source below is the Geminga pulsar. At left, the region 20 days before the flare; at right, April 14. Credit: NASA/DOE/Fermi LAT/R. Buehler
On April 12, The Fermi Large Area Telescope, and later the AGILE craft, detected a flare about 30 times more energetic than the nebula’s normal gamma-ray output and nearly five times more powerful than previous outbursts. On April 16, an even more intense flare erupted, but within a few days, the increased activity died down.
“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. “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.”
In Nebulae similar to the Crab Nebula, the emissions are theorized to be the results of processes that interact with the rapid spin of the central neutron star. Most scientists agree that the flares originate nearly one-third of a light year (roughly 20,000 times the distance from Earth to The Sun). Efforts to precisely locate these flares have not been successful.
Since September 2010, NASA’s Chandra X-ray has monitored the nebula in order to study X-ray emissions associated with the outbursts. When the Fermi scientists shared their knowledge of a new flare, Martin Weisskopf and Allyn Tennant at NASA’s Marshall Space Flight Center, executed a series of pre-planned 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.”
Researchers theorize that the flares may occur as intense magnetic fields near the pulsar undergo restructuring. These changes can accelerate electrons to nearly the speed of light. The interaction of particles with the magnetic field results in gamma ray emissions.
The energies responsible for the emissions must be well over 100 times more than can be had with any particle accelerator on Earth. At the energies responsible, it would make them some of the highest energy electrons associated with any galactic source. Using estimates based on the intensity of the gamma rays during the “Superflares” detected in April, the best estimate for the size of the emitting region is roughly the size of our solar system.
NASA’s Fermi is an astrophysics and particle physics partnership managed by NASA’s Goddard Space Flight Center in Greenbelt, Md., and developed in collaboration with the U.S. Department of Energy, with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.
The Marshall Space Flight Center manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.
For more information, images and video, visit: http://www.nasa.gov/fermi
Source(s):NASA / Goddard Space Flight Center Press Release, Fermi Gamma-ray Space Telescope News
Ray Sanders is a Sci-Fi geek, astronomer and blogger. Currently researching variable stars at Arizona State University, he writes for Universe Today, The Planetary Society blog, and his own blog, Dear Astronomer
2012-12-04 08:05:59
Source: http://www.dearastronomer.com/2011/05/12/superflares-in-crab-nebula/
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