On Electrostatic Positron Acceleration in the Accretion Flow onto Neutron Stars

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Abstract
As first shown by Shvartsman (1970), a neutron star accreting close to the Eddington limit must acquire a positive charge in order for electrons and protons to move at the same speed. The resulting electrostatic field may contribute to accelerating positrons produced near the star surface in conjunction with the radiative force. We reconsider the balance 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 exceeds ≈ 400 keV. The electrostatic field alone may produce energies of ≈ 50 keV at most. We also show that Coulomb collisions and annihilation with accreting electrons severely limit the number of positrons that escape to infinity.
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