The Neutron Star Mass Distribution
  In recent years, the number of pulsars with secure mass measurements has
increased to a level that allows us to probe the underlying neutron star mass
distribution in detail. We critically review radio pulsar mass measurements and
present a detailed examination through which we are able to put stringent
constraints on the underlying neutron star mass distribution. For the first
time, we are able to analyze a sizable population of neutron star-white dwarf
systems in addition to double neutron star systems with a technique that
accounts for systematically different measurement errors. We find that neutron
stars that have evolved through different evolutionary paths reflect
distinctive signatures through dissimilar distribution peak and mass cutoff
values. Neutron stars in double neutron star and neutron star-white dwarf
systems show consistent respective peaks at 1.35 Msun and 1.50 Msun which
suggest significant mass accretion (Delta m~0.15 Msun) has occurred during the
spin up phase. The width of the mass distribution implied by double neutron
star systems is indicative of a tight initial mass function while the inferred
mass range is significantly wider for neutron stars that have gone through
recycling. We find a mass cutoff at 2 Msun for neutron stars with white dwarf
companions which establishes a firm lower bound for the maximum neutron star
mass. This rules out the majority of strange quark and soft equation of state
models as viable configurations for neutron star matter. The lack of truncation
close to the maximum mass cutoff suggests that the 2 Msun limit is set by
evolutionary constraints rather than nuclear physics or general relativity, and
the existence of rare super-massive neutron stars is possible.
