What can the braking indices tell us about the nature of pulsars?

As a result of observational difficulties, braking indices of only six rotation-powered pulsars are obtained with some certainty. All these values are remarkably smaller than the value (n = 3) expected for pure magnetodipole radiation model. Various models have been proposed to explain this phenomenon such as introducing additional torques by particle outflow or accretion disk, changes in inclination angle or magnetic field strength, and field-reconnection process in pulsar magnetosphere. We re-examined the braking index based on previous research and suggest a constant gap potential drop in the open-field line region for pulsars with magnetospheric activity. New constraints on model parameters from observed braking indices are presented.