We will present our study of the flux and spectral variability of the Crab
above 100 MeV on different timescales ranging from days to weeks. In addition
to the four main intense and day-long flares detected by AGILE and Fermi-LAT
between Sept. 2007 and Sept. 2012, we find evidence for week-long and less
intense episodes of enhanced gamma-ray emission that we call "waves".
Statistically significant "waves" show timescales of 1-2 weeks, and can occur
by themselves or in association with shorter flares. The Sept. - Oct. 2007
gamma-ray enhancement episode detected by AGILE shows both "wave" and flaring
behavior. We extend our analysis to the publicly available Fermi-LAT dataset
and show that several additional "wave" episodes can be identified. We discuss
the spectral properties of the September 2007 "wave"/flare event and show that
the physical properties of the "waves" are intermediate between steady and
flaring states. Plasma instabilities inducing "waves" appear to involve spatial
distances l∼1016cm and enhanced magnetic fields B∼(0.5−1)}mG. Day-long flares are characterized by smaller distances and larger
local magnetic fields. Typically, the deduced total energy associated with the
"wave" phenomenon (Ew∼1042erg, where Ew is the kinetic
energy of the emitting particles) is comparable with that associated to the
flares, and can reach a few percent of the total available pulsar spindown
energy. Most likely, flares and waves are the product of the same class of
plasma instabilities that we show acting on different timescales and radiation
intensities.Comment: 2012 Fermi Symposium proceedings - eConf C12102