Fermi Gamma-ray Space Telescope

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

Paul Kin-Hang YEUNG
(Paul Kin-Hang Yeung)


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 uniform-disk radius of the Crab Nebula to be $0.040^circpm0.002^circ$ ($2.40'pm 0.12'$) in the 5--500 GeV band. We report that the systematic uncertainties associated with the PSF are not a serious issue, based on our evaluation of it with the point-like source Mkn 421. By comparison between emph{Fermi} LAT and H.E.S.S. results, we find evidence for an energy-dependent shrinking of the Crab Nebula's $gamma$-ray extension. In a more detailed modeling approach, it is necessary to include the spatial distribution of additional seed photon fields, including the emission of the dusty plasma in which the synchrotron nebula is embedded.