Evidence for 'thermal' X-rays from the Crab Nebula

Abstract

Data obtained during a recent lunar occultation are presented which indicate the existence of a weak soft X-ray source close to or within the Crab Nebula. Several interpretations of the weak source are considered, including a weak point source in the field of view, scattering of X-rays by interstellar dust along the line of sight, and an extended source associated with the expanding filaments of the nebula. The data are analyzed in terms of each model, and it is found that the source should be placed within the nebula itself. Based on this result, it is suggested that the nebular X-ray emission originates in two areas: a strong nonthermal source surrounding the pulsar and an extended 'thermal' source associated with the expanding filaments and the outer regions of the radio source. It is noted that the predicted luminosity and spectrum of the thermal source are compatible with the X-ray characteristics of other young supernova remnants, specifically Cas A and Tycho.