On the Orbital Properties of Millisecond Pulsar Binaries

We report a detailed analysis of the orbital properties of a binary millisecond pulsar (MSP) with a white dwarf (WD) companion. Positive correlations between the orbital period P b and eccentricity are found in two classes of MSP binaries with a He WD and with a CO/ONeMg WD, though their trends are different. The distribution of P b is not uniform. Deficiency of sources at P b ~ 35–50 days (Gap 1) have been mentioned in previous studies. On the other hand, another gap at P b ~ 2.5–4.5 days (Gap 2) is identified for the first time. Inspection of the relation between P b and the companion masses M c revealed the subpopulations of MSP binaries with a He WD separated by Gap 1, above which P b is independent of M c (horizontal branch) but below which P b correlates strongly with M c (lower branch). A distinctive horizontal branch and a lower branch separated by Gap 2 were identified for the MSP binaries with a CO/ONeMg WD at shorter P b and higher M c. Generally, M c are higher in the horizontal branch than in the lower branch for the MSP binaries with a He WD. These properties can be explained in terms of a binary orbital evolution scenario in which the WD companion was ablated by a pulsar wind in the post-mass-transfer phase.