  The Large Area Telescope on board the Fermi satellite observed a gamma-ray
flare in the Crab nebula lasting for approximately nine days in April of 2011.
The source, which at optical wavelengths has a size of ~11 ly across, doubled
its gamma-ray flux within eight hours. The peak photon flux was (186 +- 6) 10-7
cm-2 s-1 above 100 MeV, which corresponds to a 30-fold increase compared to the
average value. During the flare, a new component emerged in the spectral energy
distribution, which peaked at an energy of (375 +- 26) MeV at flare maximum.
The observations imply that the emission region was likely relativistically
beamed toward us and that variations in its motion are responsible for the
observed spectral variability.
