Regimes of Pulsar Pair Formation and Particle Energetics

Abstract

We investigate the conditions required forthe production of electron-positron pairs above a pulsar polar cap(PC) and the influence of pair production on the energetics of theprimary particle acceleration. Assuming space charge-limited flowacceleration including the inertial frame-dragging effect, we allowboth one-photon and two-photon pair production by either curvatureradiation (CR) photons or photons resulting from inverse Comptonscattering of thermal photons from the PC by primary electrons. Wefind that while only the younger pulsars can produce pairs throughCR, nearly all known radio pulsars are capable of producing pairsthrough nonresonant inverse Compton scatterings. The effect of theneutron star equations of state on the pair death lines is explored.We show that pair production is facilitated in more compact starsand more massive stars. Therefore, accretion of mass by pulsars inbinary systems may allow pair production in most of the millisecondpulsar population. We also find that two-photon pair production may beimportant in millisecond pulsars if their surface temperatures areabove ~=three million kelvins. Pulsars that produce pairs throughCR will have their primary acceleration limited by the effect ofscreening of the electric field. In this regime, the high-energyluminosity should follow an L HE ~E 1/2 rot dependence. The acceleration voltage drop in pulsars that producepairs only through inverse Compton emission will not be limited byelectric field screening. In this regime, the high-energy luminosityshould follow an L HE ~E rot dependence. Thus,older pulsars will have significantly lower gamma-ray luminosity.