Pulsar Wind Nebulae and the X-Ray Emission of Non-Accreting Neutron   Stars
  The general properties of the non-thermal non-pulsed X-ray emission of rotation powered pulsars are investigated in the context of a pulsar wind nebula model. An examination of the observed X-ray emission from a sample of 23 pulsars from ASCA in the energy range between 2-10 keV reveals that the relation of X-ray luminosity, $L_x$, to the pulsar spin down power, $\dot{E}$, is steeper for the non-pulsed component than for the pulsed component. Specifically, $L_{x}^{npul} \propto \dot{E}^{1.4 \pm 0.1}$ for the non-pulsed component, whereas $L_{x}^{pul} \propto \dot{E}^{1.2 \pm 0.08}$ for the pulsed component. The former relation is consistent with emission from a pulsar wind nebula model in which $L_{x}^{npul} \propto \dot{E}^{p/2}$ where $p$ is the power law index of the electron energy distribution. The relation for the pulsed component, on the other hand, is consistent with a magnetospheric emission model. 
K. S. Cheng, Ronald E. Taam, and W. Wang