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 in the energy range between 2-10 keV reveals that the relation of X-ray luminosity, Lx, to the pulsar spin down power, Ė, is steeper for the non-pulsed component than for the pulsed component. Specifically, L x ∝ Ė 1.4±0.1 for the non-pulsed component, whereas L x ∝ Ė 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 ∝ Ė 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, Γ, was found to be correlated to the conversion efficiency of spin down power to non-pulsed X-ray emission with greater efficiencies for Γ ∼ 2 − 2.5 than for Γ ∼ 1.5 − 2. Such a relation is 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 and 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 non-thermal symmetric emission morphology (point-like) and elongated emission morphology (tail-like) from sources similar to Geminga and PSR B1757-24. Subject headings: stars: neutron – X-rays: stars – pulsars: general – radiation mechanisms: non-thermal