The Energy-dependent $\gamma$-ray Morphology of the Crab Nebula Observed with the Fermi Large Area Telescope

Abstract

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 un-pulsed emission at GeV to 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 LAT data accumulated over $\sim$9.1 years with a properly refined model for the Crab pulsar's spectrum, we determined the 68\% containment radius ($R_{68}$) of the Crab Nebula to be $({0.0330\pm0.0025_{stat}}{^{+0.0012}_{-0.0075}}_{sys})^\circ$ (${1.98'\pm0.15'_{stat}}{^{+0.07'}_{-0.45'}}_{sys}$) 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 \emph{Fermi} LAT and H.E.S.S. results on the spatial extension, we find evidence for an energy-dependent shrinking of the Crab Nebula's $\gamma$-ray extension ($R_{68}\propto E_\mathrm{IC}^{-\alpha}$ where $\alpha=0.155\pm0.035_{stat}{-0.037}_{sys}$).