Introduction This chapter deals with X-ray emission from isolated neutron stars for which the energy for the observed X-rays is thought to originate from the rotation of the neutron star, or from an internal heat reservoir following formation. Rotation power can manifest itself as pulsed emission, or as nebular radiation produced by a relativistic wind of particles emitted by the neutron star. Residual heat of formation is observed as soft X-ray emission from young neutron stars. Such thermal radiation, however, can also be produced as a result of reheating from internal or external sources. Rotation-powered pulsed and nebular X-ray emission, as well as thermal emission, can often be observed in a single object simultaneously; this is both fascinating and annoying, as one invariably contaminates the study of the other. There are also a handful of neutron stars for which the origin of the observed X-ray emission is unclear but may be related to the above processes; we will discuss those as well. Rotation-powered neutron stars are generally referred to as “radio pulsars” since it is at radio wavelengths that the vast majority of the catalogued population (currently numbering ∼1400) is observed. However, the radio emission is energetically unimportant, and we now know of several rotation-powered neutron stars that are not detected as radio sources in spite of deep searches (e.g. Crawford et al . 1998; McLaughlin et al . 2001). We therefore use the more physically motivated term “rotation-powered.”
