Fermi Large Area Telescope observations of the fast-dimming Crab Nebula in 60–600 MeV

Abstract

Context. The Crab pulsar and its nebula are the origin of relativistic electrons which can be observed through their synchrotron and inverse Compton emission. The transition between synchrotron-dominated and inverse-Compton-dominated emissions takes place at ≈109 eV. Aims. The short-term (lasting for one week to months) flux variability of the synchrotron emission from the most energetic electrons is investigated with data from ten years of observations with the Fermi Large Area Telescope in the energy range from 60 MeV to 600 MeV. Methods. We reconstructed the off-pulse light curve reconstructed from phase-resolved data. The corresponding histogram of flux measurements was used to identify distributions of flux-states and the statistical significance of a lower-flux component was estimated with dedicated simulations of mock light curves. The energy spectra for different flux states were also reconstructed. Results. We confirm the presence of flaring-states which follow a log-normal flux distribution. Additionally, we discovered a low-flux state where the flux drops to as low as 18.4% of the intermediate-state average flux and remains there for several weeks. The transition time is observed to be as short as two days. The energy spectrum during the low-flux state resembles the extrapolation of the inverse-Compton spectrum measured at energies beyond several GeV energy, implying that the high-energy part of the synchrotron emission is dramatically depressed. Conclusions. The low-flux state found here and the transition time of at most ten days indicate that the bulk (>75%) of the synchrotron emission above 108 eV originates in a compact volume with apparent angular size of θ ≈ 0″​​.4 tvar/(5 d). We tentatively infer that the so-called inner knot feature is the origin of the bulk of the γ-ray emission.