We present the first optical spectroscopy of five confirmed ( or strong candidate ) redback millisecond pulsar binaries , obtaining complete radial velocity curves for each companion star .
The properties of these millisecond pulsar binaries with low-mass , hydrogen-rich companions are discussed in the context of the 14 confirmed and 10 candidate field redbacks .
We find that the neutron stars in redbacks have a median mass of 1.78 \pm 0.09 M _ { \odot } with a dispersion of \sigma = 0.21 \pm 0.09 .
Neutron stars with masses in excess of 2 M _ { \odot } are consistent with , but not firmly demanded by , current observations .
Redback companions have median masses of 0.36 \pm 0.04 M _ { \odot } with a scatter of \sigma = 0.15 \pm 0.04 M _ { \odot } , and a tail possibly extending up to 0.7– 0.9 M _ { \odot } .
Candidate redbacks tend to have higher companion masses than confirmed redbacks , suggesting a possible selection bias against the detection of radio pulsations in these more massive candidate systems .
The distribution of companion masses between redbacks and the less massive black widows continues to be strongly bimodal , which is an important constraint on evolutionary models for these systems .
Among redbacks , the median efficiency of converting the pulsar spindown energy to \gamma -ray luminosity is \sim 10 \% .