High‐Energy Emission from Millisecond Pulsars

The X-ray and gamma-ray spectrum of rotation-powered millisecond pulsars isinvestigated in a model for acceleration and pair cascades on open field linesabove the polar caps. 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 theaccelerating electric field. The accelerating particles maintain high Lorentzfactors and undergo cyclotron resonant absorption of radio emission, thatproduces and maintains a large pitch angle, resulting in a strong synchrotroncomponent. The resulting spectra consist of several distinct components:curvature radiation from primary electrons dominating from 1 - 100 GeV,synchrotron radiation from primary and secondary electrons dominating up toabout 100 MeV, and much weaker inverse-Compton radiation from primary electronsat 0.1 - 1 TeV. We find that the relative size of these components depends onpulsar period, period derivative, and neutron star mass and radius with thelevel of the synchrotron component also depending sensitively on the radioemission properties. This model is successful in describing the observed X-rayand gamma-ray spectrum of PSR J0218+4232 as synchrotron radiation, peakingaround 100 MeV and extending up to a turnover around several GeV. The predictedcurvature radiation components from a number of millisecond pulsars, as well asthe collective emission from the millisecond pulsars in globular clusters,should be detectable with AGILE and GLAST. We also discuss a hidden populationof X-ray-quiet and radio-quiet millisecond pulsars which have evolved below thepair death line, some of which may be detectable by telescopes sensitive above1 GeV.Comment: 34 pages, 6 figures, accepted for publication in Astrophysical Journa