Neutron Star Mass Measurements. I. Radio Pulsars

There are now about fifty known radio pulsars in binary systems, including atleast five in double neutron star binaries. In some cases, the stellar massescan be directly determined from measurements of relativistic orbital effects.In others, only an indirect or statistical estimate of the masses is possible.We review the general problem of mass measurement in radio pulsar binaries, andcritically discuss all current estimates of the masses of radio pulsars andtheir companions. We find that significant constraints exist on the masses oftwenty-one radio pulsars, and on five neutron star companions of radio pulsars.All the measurements are consistent with a remarkably narrow underlyinggaussian mass distribution, m = 1.35 +- 0.04 solar masses. There is no evidencethat extensive mass accretion (delta m >~ 0.1 solar mass) has occurred in thesesystems. We also show that the observed inclinations of millisecond pulsarbinaries are consistent with a random distribution, and thus find no evidencefor either alignment or counteralignment of millisecond pulsar magnetic fields.