  We have studied the X-ray properties of ageing historical core-collapse
supernovae in nearby galaxies, using archival data from Chandra, XMM-Newton and
Swift. We found possible evidence of a young X-ray pulsar in SN 1968D and in
few other sources, but none more luminous than ~ a few 10^{37} erg/s. We
compared the observational limits to the X-ray pulsar luminosity distribution
with the results of Monte Carlo simulations for a range of birth parameters. We
conclude that a pulsar population dominated by periods <~ 40 ms at birth is
ruled out by the data.
