Pulsar wind nebulae and the nonthermal X-ray emission of millisecond pulsars

The nonthermal, nonpulsed X-ray emission of MSPs is investigated. As in young pulsars, MSPs emit a relativistic wind, which in interacting with the ISM and/or a binary companion can significantly contribute to the nonpulsed emission of these pulsars. An application and extension of a simple model developed for young pulsars is applied to the old recycled MSP B1957+20. It is found that the pulsar wind can indeed contribute to both the resolved and unresolved X-ray emission. For other MSPs in the Galactic field for which the spectral index of the nonpulsed component has been measured (i.e., PSR B1937+21, PSR J0218+4232) the contribution of the pulsar wind to the nonpulsed X-ray luminosity is estimated. For the MSPs in the core regions of globular clusters, the pulsar wind nebula is likely affected by its interaction with the dense stellar environment, possibly leading to a diminished contribution to the total X-ray emission. In this case, the existence of nonthermal nonpulsed X-ray emission is more likely for binary than for isolated MSPs, with the emission arising from the interaction of the relativistic pulsarwind and a binary companion. Our study suggests that the magnetization parameter in the pulsar wind nebulae of MSPs is significantly larger than that of the Crab Nebula, by about a factor of 10. The nebulae powered by rapidly rotating neutron stars either in isolation or in the quiescent state of soft X-ray transients can contribute to the faint X-ray source population associated with onaccreting neutron stars. The emission from MSPs moving at high velocities (greater than or similar to 100 km s(-1)) through regions of the ISM characterized by magnetic field strengths less than or similar to 0.1 mG may appear spatially extended with a tail-like morphology. Thus, MSPs may also contribute to the faint filamentary X-ray source subpopulation in the Galaxy.