The stopping of fast protons in the accretion column of a neutron star

New calculations have been performed of the Fokker–Planck coefficients for a proton in an accretion plasma. The resulting Fokker–Planck equation is solved numerically to find the stopping length of the proton. For an infall velocity of c/4 into a plasma of temperature 5 keV and density 10²³ particle cm⁻³, only 0.6 g cm⁻² are required to stop the proton. This result arises because the gyro radius, λ_g, of the proton is smaller than the Debye screening length, λ_D, of the plasma. Previous authors have used techniques valid when $$\lambda_\text g\gg \lambda_\text D$$ and have calculated stopping lengths greatly in excess of those calculated here.