AGES OF PULSARS IN SUPERNOVA-REMNANTS AND THE ORIGIN OF NEUTRON-STAR MAGNETIC-FIELDS

The true ages of pulsars can, at present, only be found for those cases where the age of the associated supernova remnant can be determined. Recent measurements of the Vela supernova remnant imply age estimates for the Vela pulsar which are discrepant with its spin-down age. We show that this is not surprising and that the spin-down age of the Vela pulsar (PSR 0833 - 45) must in general differ from its true age. This is because it is virtually certain that the surface magnetic field B-S will not be a constant, but will vary on time-scales characteristic of young supernova remnants. This is valid for young pulsars regardless of the mechanism for the origin and evolution of their magnetic fields. We also demonstrate that, although the surface field of the pulsar may have grown from the time that it was born, one cannot determine from the current observations whether the pulsar was born with a strong surface field or whether the field was produced after birth.