Transient Gamma-ray flare from the Crab
Credit: ASI, NASA

What's Eating the Crab?

The Crab Nebula, the remains of a star which exploded in A. D. 1054, is regarded by astronomers as one of the steadiest sources in the sky. So steady, in fact, that it's used as a standard source against which nearly all high energy satellite observatories are measured and calibrated. But recently something seems to be bothering the Crab. A surprising measurement of the Crab by the Agenzia Spaziale Italiana's AGILE Gamma-ray observatory (shown on the lower left in the above image of the Crab Nebula) found that the Crab varied in its gamma-ray output by a factor of 3 in September 2010. This change was also confirmed by NASA's Fermi Gamma-ray Space Telescope (superimposed above, lower right) as well. Astronomers suspected something strange going on with the central pulsar (a rapidly spinning neutron star) that powers the emission from the nebula, but followup observations using the Hubble Space Telescope and the Chandra X-ray observatory showed no real change in the pulsar in optical or X-ray wavelengths, and gamma-ray emissions showed it pulsing regularly at about 30 times per second as it nearly always does. So why is the Crab so crabby? Astronomers think the gamma-ray flare might have been produced by a strong shock wave moving through the nebula at near the speed of light.
Published: February 28, 2011

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Page Author: Dr. Michael F. Corcoran
Last modified Monday, 07-Mar-2011 07:00:49 EST