How do we know that pulsars must be neutron stars?
a) We have observed massive-star supernovae produce pulsars.
b) This is only a theory that has not yet been confirmed by observations.
c) No massive object, other than a neutron star, could spin as fast as we observe pulsars to spin and remain intact.
d) Pulsars have the same upper mass limit as neutron stars do.
e) Telescopic images of pulsars and neutron stars look exactly the same.