On the Formation of Inner Vacuum Gaps in Radio Pulsars

The problem of formation of the Ruderman-Sutherland-type inner vacuum gap in neutron stars with Ω˙B<0 is considered. It is argued by means of the condition T_i/T_s>1 (where T_i is the critical temperature above which ⁵⁶₂₆Fe ions will not be bound at the surface and T_s is the actual temperature of the polar cap surface heated by the back flow of relativistic electrons) that the inner vacuum gap can form, provided that the actual surface magnetic field is extremely strong (B_s>~10¹³ G) and curved (R<10⁶ cm), irrespective of the value of dipolar component measured from the pulsar spin-down rate. Calculations are carried out for pulsars with drifting subpulses and/or periodic intensity modulations, in which the existence of the quasi-steady vacuum gap discharging via EXB drifting sparks is almost unavoidable. Using different pair-production mechanisms and different estimates of the cohesive energies of surface iron ions, we show that it is easier to form the vacuum gap controlled by the resonant inverse Compton scattering seed photons than by the curvature radiation seed photons.