Pulsar wind nebulae and the X-ray emission of nonaccreting neutron stars

The general properties of the nonthermal nonpulsed 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 in the energy range between 2 and 10 keV reveals that the relation of X-ray luminosity, L-X, to the pulsar spin-down power, (E) over dot, is steeper for the nonpulsed component than for the pulsed component. Specifically, L-X(npul) proportional to(E) over dot (1.4 +/- 0.1) for the nonpulsed component, whereas L-X(pul) proportional to (E) over dot (1.2 +/- 0.08) for the pulsed component. The former relation is consistent with emission from a pulsar wind nebula model in which L-X(npul) proportional to (E) over dot (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. In addition, the photon spectral index, Gamma, was found to be correlated with the conversion efficiency of spin-down power into nonpulsed X-ray emission, with greater efficiencies for Gamma similar to 2-2.5 than for Gamma similar to 1.5-2. Such a relation can be naturally understood within the framework of a pulsar wind nebula model, with the former relation corresponding to the emission of X-rays in the fast-cooling regime and the latter relation corresponding to emission in the slow-cooling regime. The X-ray properties of pulsar wind nebulae are sensitive to the physical conditions ( e. g., the density and magnetic field) of the interstellar medium, which can lead to important differences between the X-ray emission characteristics ( luminosity, photon spectral index, and emission morphology) of pulsars in various environments. Such wind nebulae can contribute to the nonthermal symmetric emission morphology (pointlike) and elongated emission morphology (tail-like) from sources similar to Geminga and PSR B1757 - 24.