Gamma-ray flux depressions of the Crab Nebula in the high-energy range

Abstract

The giant gamma-ray flares of the Crab Nebula discovered by AGILE and Fermi observatories came as a surprise and have challenged the existing models of pulsar wind nebulae. We have carried out an analysis of 10.5 yr of Fe r m i-LAT observations (August 2008 to February 2019) and investigated variability of the Crab Nebula in the 100-300 MeV range. Besides the flares, we found several month long depressions of the gamma-ray flux and identified several cases of sharp flux drops, where during 1 week the flux decreased by an order of magnitude with respect to its average value. No statistically significant variations of the nebula flux in the E >10 GeV range were found in the data. We discuss possible implications of the observed gamma-ray flux depressions on the model of synchrotron emission of the Crab Nebula.