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Context: The Crab pulsar and its nebula are the origin of relativisticelectrons which can be observed through their synchrotron and inverse Comptonemission. The transition between synchrotron-dominated andinverse-Compton-dominated emissions takes place at $\approx 10^9$ eV. Aims: Theshort-term (weeks to months) flux variability of the synchrotron emission fromthe most energetic electrons is investigated with data from ten years ofobservations with the Fermi Large Area Telescope (LAT) in the energy range from60 MeV to 600 MeV. Methods: The off-pulse light-curve has been reconstructedfrom phase-resolved data. The corresponding histogram of flux measurements isused to identify distributions of flux-states and the statistical significanceof a lower-flux component is estimated with dedicated simulations of mocklight-curves. The energy spectra for different flux states are reconstructed.Results: We confirm the presence of flaring-states which follow a log-normalflux distribution. Additionally, we discover a low-flux state where the fluxdrops to as low as 18.4% of the intermediate-state average flux and stays therefor several weeks. The transition time is observed to be as short as 2 days.The energy spectrum during the low-flux state resembles the extrapolation ofthe inverse-Compton spectrum measured at energies beyond several GeV energy,implying that the high-energy part of the synchrotron emission is dramaticallydepressed. Conclusions: The low-flux state found here and the transition timeof at most 10 days indicate that the bulk ($>75$%) of the synchrotron emissionabove $10^8$ eV originates in a compact volume with apparent angular size of$\theta\approx0.4" t_\mathrm{var}/(5 \mathrm{d})$. We tentatively infer thatthe so-called inner knot feature is the origin of the bulk of the $\gamma$-rayemission.

Fermi Large Area Telescope Observations of the Fast-dimming Crab Nebula in 60-600 MeV