Torque decay in the pulsar diagram

Effects of crustal ohmic dissipation and alignment

¹ Nordic Institute for Theoretical Physics (NORDITA), Blegdamsvej 17, 2100 Copenhagen Ø, Denmark
² Inter-University Centre for Astronomy and Astrophysics, Pune 4111007, India e-mail: sushan@iucaa.ernet.in

Corresponding author: T. M. Tauris, tauris@nordita.dk

Received: 30 January 2001
Accepted: 5 July 2001

Abstract

We investigate the evolution of pulsars in the diagram. We first present analytical formulae to follow the evolution of a pulsar using simple exponential models for magnetic field decay and alignment. We then compare these evolutionary tracks with detailed model calculations using ohmic decay of crustal neutron star magnetic fields. We find that, after an initial phase with a small braking index, n, pulsars evolve with enhanced torque decay () 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 their observed characteristic age, . It is not possible from data to constrain model parameters of the neutron star crust.