Electron Energy Losses Near Pulsar polar Caps

Abstract

We present results of a model for the energetics of electrons accelerated by the large electric fields generated by a rotating highly magnetized neutron star. The energy loss mechanisms we consider in our calculations include magnetic Compton scattering of thermal x-ray photons, triplet pair production, and curvature radiation emission. The electron acceleration mechanism is assumed to operate only to a height above the polar cap approximately equal to the polar cap radius. We find several interesting results. First, magnetic Compton scattering is the dominant energy loss process when the electron Lorentz factors are < few x10^6 for typical gamma-ray pulsar magnetic fields and surface temperatures measured by ROSAT. The amount of energy converted to photons by accelerated electrons ranges from ~10% to ~100% of gamma_0 mc^2 where gamma_0 is the maximum Lorentz factor an electron can attain with no radiative losses. We also find that if B > 10^13 G and T > 3x10^6 K, the Lorentz factors of the electrons can be limited to values < 10^3 assuming values for the size of the neutron star thermal polar cap comparable to the polar cap size determined by the open field lines. Such small Lorentz factors may be capable of explaining the gamma-ray emission from PSR 1509-58 which is observed only at energies <1 MeV. We calculated the fraction of the electron's kinetic energy that is converted to gamma rays for the three gamma-ray pulsars which show thermal x-ray spectra, namely Vela (60%), Geminga (5%), and PSR 1055-52 (20%).