High‐Energy Emission from Millisecond Pulsars

Abstract

The X-ray and gamma-ray spectrum of rotation-powered millisecond pulsars is\ninvestigated in a model for acceleration and pair cascades on open field lines\nabove the polar caps. Although these pulsars have low surface magnetic fields,\ntheir short periods allow them to have large magnetospheric potential drops,\nbut the majority do not produce sufficient pairs to completely screen the\naccelerating electric field. The accelerating particles maintain high Lorentz\nfactors and undergo cyclotron resonant absorption of radio emission, that\nproduces and maintains a large pitch angle, resulting in a strong synchrotron\ncomponent. The resulting spectra consist of several distinct components:\ncurvature radiation from primary electrons dominating from 1 - 100 GeV,\nsynchrotron radiation from primary and secondary electrons dominating up to\nabout 100 MeV, and much weaker inverse-Compton radiation from primary electrons\nat 0.1 - 1 TeV. We find that the relative size of these components depends on\npulsar period, period derivative, and neutron star mass and radius with the\nlevel of the synchrotron component also depending sensitively on the radio\nemission properties. This model is successful in describing the observed X-ray\nand gamma-ray spectrum of PSR J0218+4232 as synchrotron radiation, peaking\naround 100 MeV and extending up to a turnover around several GeV. The predicted\ncurvature radiation components from a number of millisecond pulsars, as well as\nthe collective emission from the millisecond pulsars in globular clusters,\nshould be detectable with AGILE and GLAST. We also discuss a hidden population\nof X-ray-quiet and radio-quiet millisecond pulsars which have evolved below the\npair death line, some of which may be detectable by telescopes sensitive above\n1 GeV.\n