Condensed Surfaces of Magnetic Neutron Stars and Particle Acceleration Above Pulsar Polar Caps

摘要

Recent calculations indicate that the cohesive energy of condensed matter increases with magnetic field strength and becomes very significant at magnetar-like fields (e.g., 10 keV at 3×1014 G for zero-pressure condensed iron). This implies that for sufficiently strong magnetic fields and/or low temperatures, the neutron star surface may be in a condensed state with little gas or plasma above it. Such surface condensation may lead to the formation of a charge-depleted acceleration zone (“vacuum gap”) in the magnetosphere above the stellar polar cap. Using the latest results on the cohesive property of magnetic condensed matter, we quantitatively determine the conditions for vacuum gap formation in magnetic neutron stars. We also study the physics of pair cascades in the (Ruderman-Sutherland type) vacuum gap model for photon emission by accelerating electrons and positrons due to both curvature radiation and resonant/nonresonant inverse Compton scattering. Our calculations of the condition of cascade-induced vacuum breakdown and the related pulsar death line/boundary eneralize previous works to the superstrong field regime.