Gamma-Ray Activity in the Crab Nebula: The Exceptional Flare of April 2011.

摘要

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 approximately 11 ly across, doubled its gamma-ray flux within eight hours. The peak photon flux was (186 +/- 6) x 10(exp -7) /square cm/s 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 relativistically beamed toward us and that variations in its motion are responsible for the observed spectral variability.