"For 40 years, most astronomers regarded the Crab as a standard candle," said Colleen Wilson-Hodge, an astrophysicist at NASA's Marshall Space Flight Center in Huntsville, Ala., who presented the findings today at the American Astronomical Society meeting in Seattle. "Now, for the first time, we're clearly seeing how much our candle flickers."
Fermi's other instrument, the Large Area Telescope, has detected unprecedented gamma-ray flares from the Crab, showing that it is also surprisingly variable at much higher energies. A study of these events was published Thursday, Jan. 6, in Science Express.
The nebula's power comes from the central neutron star, which is also a pulsar that emits fast, regular radio and X-ray pulses. This pulsed emission exhibits no changes associated with the decline, so it cannot be the source. Instead, researchers suspect that the long-term changes probably occur in the nebula's central light-year, but observations with future telescopes will be needed to know for sure.
"This environment is dominated by the pulsar's magnetic field, which we suspect is organized precariously," 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. "The X-ray changes may involve some rearrangement of the magnetic field, but just where this happens is a mystery."
The Crab Nebula is a supernova remnant located 6,500 light-years away in the constellation Taurus.
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 GBM Instrument Operations Center is located at the National Space Science Technology Center in Huntsville, Ala. The team includes a collaboration of scientists from UAH, NASA's Marshall Space Flight Center in Huntsville, the Max Planck Institute for Extraterrestrial Physics in Germany and other institutions.
NASA Goddard manages Swift, RXTE and a guest observer facility for U.S. participation in the European Space Agency's INTEGRAL mission.
Download related video and visuals from NASA Goddard's Scientific Visualization Studio
NASA's Goddard Space Flight Center, Greenbelt, Md.
NASA's Marshall Space Flight Center, Huntsville, Ala.