The physical nature of ultraluminous X-ray sources (ULXs) is not well understood. Accreting black holes perhaps more massive than those in Galactic X-ray binaries are thought to power most ULXs. But could any of them be young X-ray pulsars, instead? Pulsars are powered by spin-down luminosity, hence they are not Eddington-limited. Their X-ray spectra have a dominant power-law component plus a soft excess, like many ULXs. From the observed properties of old radio pulsars, it was suggested that typical pulsar birth parameters are B ~ a few E12 - 1E13 Gauss, and initial Period ~ 10 ms. If so, most young pulsars should have X-ray luminosities ~ 1E38-1E40 erg/s, at least for the first few decades or centuries. We searched for associations between bright X-ray sources and historical supernovae (ages ~ 10-100 yrs). However, none of the historical supernovae contains such a bright pulsar. Why? And what does this tell us about neutron star formation in a supernova?

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