Nature of the crablike, pulsar-powered supernova remnant 0540-693: the pulsar's initial period

While it is generally accepted that most normal pulsars are not born rotating near the dynamical limit of about 1 ms, there is almost no direct information on the initial periods of pulsars, except for the Crab pulsar where P0 approx. 10-20 ms. Radio, optical and x-ray observations of the Crablike supernova remnant in the Large Magellanic Cloud, 0540-693, are used to deduce the initial properties of its recently discovered 50 ms x-ray pulsar. Simple evolutionary considerations give the age of the object as 700-1000 years. Its appearance in all frequency bands can be understood if it is a combination of emission resulting from several solar masses of fast-moving (approx. 104 km/s) material interacting with the surrounding medium, producing the ratio emission, and a solar mass or so of processed material which has been swept into a shell and accelerated by the pulsar. The original luminosity of the pulsar is restricted to the range (5-20) x 1038 erg/s, somewhat less luminous than the Crab pulsar at birth. The original rotational energy is inferred to be 2 x 1049 erg, depending only insensitively on model details; this implies an initial period for the pulsar of 34 +- 5 ms. Thus this pulsar, the second for which initial parameters can be inferred, joins the Crab in having a relatively slow rotation at birth. The significance of this finding for theories of pulsars and their supernova remnants is discussed. 13 references