http://arxiv.org/abs/1903.07527
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 $\alpha=0.155\pm0.035_{stat}{-0.037}_{sys}$).
P. Yeung and D. Horns
Tue, 19 Mar 19
41/100
Comments: Accepted for publication in ApJ
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