Torque decay in the pulsar (P,(P)over-circle) diagram - Effects of crustalohmic dissipation and alignment

We investigate the evolution of pulsars in the (P, (P) over dot P) diagram. We first present analytical formulae to follow the evolution of a pulsar u sing simple exponential models for magnetic field decay and alignment. We t hen compare these evolutionary tracks with detailed model calculations usin g ohmic decay of crustal neutron star magnetic fields. We find that, after an initial phase with a small braking index, n, pulsars evolve with enhance d torque decay (n >> 3) for about 1 Myr. The long term evolution depends on the impurity parameter of the crust. If impurities are negligible in older isolated pulsars we expect their true age to be approximately equal to the ir observed characteristic age, tau = P/(2(P) over dot). It is not possible from data to constrain model parameters of the neutron star crust.