2019ApJ...875..123Y

The Crab Nebula is a bright emitter of non-thermal radiation across the entire accessible range of wavelengths. The spatial and spectral structures of the synchrotron nebula are well-resolved from radio to hard X-ray emission. The unpulsed emission at GeV-TeV energies is mostly produced via inverse-Compton scattering of energetic electrons with the synchrotron-emitted photons. The spatial structure observed at these energies provides insights into the distribution of electrons and indirectly constrains the so-far unknown structure of the magnetic field in the nebula. Analyzing the Large Area Telescope (LAT) data accumulated over ∼9.1 yr with a properly refined model for the Crab pulsar's spectrum, we determined the 68% containment radius (R₆₈) of the Crab Nebula to be (0.0330±0.0025_{stat–0.0075}^+0.0012_sys)^○ (1′ / . 98±0′ / . 15_stat–0.^′45+0_.^′07^_sys) in the 5-500 GeV band. The estimated systematic uncertainty is based on two factors: (1) different analysis methods, morphological models and event types, and (2) the point-spread function evaluated with observations of Mkn 421. When comparing the Fermi-LAT and High Energy Stereoscopic System results on the spatial extension, we find evidence for an energy-dependent shrinking of the Crab Nebula's γ-ray extension (R₆₈∝E_IC^-α^, where α=0.155±0.035_stat-0.037_sys).