The influence of pulsars on supernova remnants

Abstract

Both Supernova Remnants (SNRs) and pulsars are products of supernova explosions; so it is natural to expect an association between them. Since various selection effects work against the detection of pulsars, the best way to infer the presence of a pulsar is to detect the synchrotron nebula it is expected to produce. However, only 10% of the observed SNRs have plerionic cores. We argue that the main reason for the paucity of plerions is that most pulsars are born spinning rather slowly, with periods ? 100 ms. We suggest that the rotational energy of a newly born neutron star may be quickly extracted to power the supernova explosion, leaving behind a slowly rotating pulsar. The overall morphology of a supernova remnant depends on the relative brightness of the pulsar?produced plerionic component and the shell component resulting from the interaction with the ambient interstellar medium. Our model calculations show how the morphology of an SNR depends on its evolutionary stage, the pulsar parameters, the velocity of expansion, and the density of the ambient medium. If the external medium is of a rather low density, the lifetime of an SNR is considerably reduced. There are reasons to believe that a good fraction of SNRs may be expanding in low?density bubbles created by the high?velocity winds from their progenitor stars. If so, then the conventional estimate of the SNR birthrate requires an upward revision by a factor ? 2. Such bubbles would also prevent a smooth continuation of the radio emission of the “supernova” phase into the “remnant” phase, thus explaining the absence of SNRs younger than Cassiopeia A in our Galaxy. We also describe how the presence of a young, fast pulsar would influence the light curve of a supernova itself. Finally, from a comparison of the population of millisecond pulsars and their progenitors, namely the Low?Mass X?ray Binaries, we conclude that millisecond pulsars must live for ? 10? years, and this is possible only if their magnetic fields do not decay below a minimum value of ? 5 × 10? gauss.