High-Energy Emission From Millisecond Pulsars

A model for the X-ray and γ -ray spectrum of rotation-powered millisecond pulsars is presented. Although these pulsars have low surface magnetic fields, their short periods allow them to have large magnetospheric potential drops, but the majority do not produce sufficient pairs to completely screen the accelerating field. In these sources, the primary and secondary electrons continue to accelerate to high altitude and their Lorentz factors are limited by curvature and synchrotron radiation reaction. The accelerating particles maintain high Lorentz factors and undergo cyclotron resonant absorption of radio emission, resulting in a large gain in pitch angle. The resulting spectra consist of several distinct components: curvature radiation from primary electrons dominating from 1 - 50 GeV, synchrotron radiation from primary and secondary electrons dominating up to about 10 MeV, and much weaker inverse-Compton radiation from primary electrons at 0.1 - 1 TeV. We find that the relative size of these components depends on pulsar period and period derivative, with the level of the synchrotron component depending sensitively on the radio emission properties. This model is successful in describing the observed X-ray and γ -ray spectrum of PSR J0218+4232 as synchrotron radiation, peaking around 1-10 MeV and extending up to a steep turnover around 100 MeV. The predicted curvature radiation components from a number of millisecond pulsars, as well as the collective emission from the millisecond pulsars in globular clusters, should be detectable with AGILE and GLAST. We also discuss a hidden population of X-ray-quiet and radio-quiet millisecond pulsars which have evolved below the pair death line, some of which may be detectable by telescopes sensitive above 1 GeV.