The age of PSR J1012+5307

Lorimer et al. have recently reported that the spin-down age (similar to 7 x 10(9) yr) of the millisecond pulsar PSR J1012 + 5307 is much longer than the cooling age (3 x 10(8) yr) of its white dwarf companion. A scenario where the neutron star magnetic field decays spontaneously, and conservative accretion of all the mass lost by the secondary then 'recycles' the neutron star as a millisecond pulsar, cannot explain this discrepancy: the pulsar spins too rapidly at the end of the accretion phase to reach the current value within the white dwarf cooling time. However, if the decay of the neutron star magnetic field is induced by the accretion process itself, spin-up and field decay occur on similar time-scales, and the neutron star spin period cannot in general reach the usual short equilibrium value. The pulsar is thus able to accrete all the mass lost by the companion, reaching its current period and period derivative within the white dwarf cooling time.