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 epsilon 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 Pb is not uniform. Deficiency of sources at P-b similar to 35-50 days (Gap 1) have been mentioned in previous studies. On the other hand, another gap at P-b similar to 2.5-4.5 days (Gap 2) is identified for the first time. Inspection of the relation between Pb and the companion masses M-c revealed the subpopulations of MSP binaries with a He WD separated by Gap 1, above which Pb is independent of M-c (horizontal branch) but below which P-b correlates strongly with Mc (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.