Constraining vacuum gap models of pulsar radio emission using the intensity modulation index

Recent observations suggest that the level of pulse-to-pulse intensity modulation observed in a given radio pulsar may depend on its period and period derivative. Such a ``modulation index relationship'' (MIR) may be an important tool for determining the physical processes behind the radio emission. In the context of sparking gap models, the exact functional form of the MIR depends on the physical processes occurring on the surface of the neutron star in the region known as the ``vacuum gap.'' Several possible vacuum gap models are studied here in order to determine the expected MIR for a given model. Current observations are consistent with two of the four models studied: the curvature radiation driven vacuum gap and the curvature radiation driven near threshold vacuum gap (CR-NTVG). It is shown that the inverse Compton scattering driven vacuum gap models are not supported by the current data. Given that the current data best supports the CR-NTVG model, its possible that all pulsars have strong (approx 10^13 G) surface magnetic fields.