As a result of observational difficulties , braking indices of only six rotation-powered pulsars are obtained with certainty , all of which are remarkably smaller than the value ( n = 3 ) expected for pure magnetodipole radiation model .
This is still a real fundamental question not being well answered after nearly forty years of the discovery of pulsar .
The main problem is that we are shamefully not sure about the dominant mechanisms that result in pulsars ’ spin-down .
Based on the previous works , the braking index is re-examined , with a conclusion of suggesting a constant gap potential drop for pulsars with magnetospheric activities .
New constrains on model parameters from observed braking indices are presented .