ON ELECTROSTATIC POSITRON ACCELERATION IN THE ACCRETION FLOW ONTO NEUTRON-STARS

As first shown by Shvartsman (1970), a neutron star accreting close to the Eddington limit must acquire a positive charge in order for elect rons and protons to move at the same speed. The resulting electrostati c field may contribute to accelerating positrons produced near the sta r surface in conjunction with the radiative force. We reconsider the b alance between energy gains and losses, including inverse Compton (IC) , bremsstrahlung, and nonradiative scatterings. It is found that, even accounting for IC losses only, the maximum positron energy never exce eds approximate to 400 keV. The electrostatic field alone may produce energies of approximate to 50 keV at most. We also show that Coulomb c ollisions and annihilation with accreting electrons severely limit the number of positrons that escape to infinity.