Two Populations of X-ray Pulsars Produced by Two Types of Supernovae
  Two types of supernova are thought to produce the overwhelming majority of
neutron stars in the Universe. The first type, iron-core collapse supernovae,
occurs when a high-mass star develops a degenerate iron core that exceeds the
Chandrasekhar limit. The second type, electron-capture supernovae, is
associated with the collapse of a lower-mass oxygen-neon-magnesium core as it
loses pressure support owing to the sudden capture of electrons by neon and/or
magnesium nuclei. It has hitherto been impossible to identify the two distinct
families of neutron stars produced in these formation channels. Here we report
that a large, well-known class of neutron-star-hosting X-ray pulsars is
actually composed of two distinct sub-populations with different characteristic
spin periods, orbital periods and orbital eccentricities. This class, the
Be/X-ray binaries, contains neutron stars that accrete material from a more
massive companion star. The two sub-populations are most probably associated
with the two distinct types of neutron-star-forming supernovae, with
electron-capture supernovae preferentially producing system with short spin
period, short orbital periods and low eccentricity. Intriguingly, the split
between the two sub-populations is clearest in the distribution of the
logarithm of spin period, a result that had not been predicted and which still
remains to be explained.
