Four new millisecond pulsars in the galactic disk

We report the discovery of four millisecond pulsars as part of a conntinuing survey of the Southern Hemisphere with the Parkes radio telescope. Three of the pulsars, PSRs J0613-0200, J1455-3330, and J1643-1224, are in circular orbits with low-mass (approximately 0.1-0.3 solar mass) companions. The fourth pulsar, PSR J1730-2304, has a period of 8.12 ms and appears to be solitary. Based on the results of the Parkes and other surveys, we estimate the number of low-mass binary pulsars with luminosities greater than 2.5 mJy/sq kpc in the Galactic disk to be at least 40,000. If the lifetimes of these systems are comparable to the age of the disk then, as a group, their birthrate is consistent with that of their proposed progenitors, the low-mass X-ray binaries. The binary pulsars give some support to the relation between orbital period and companion mass discussed by Joss, Rappaport, & Lewis and the relation between orbital eccentricity and orbital period discussed by Phiney. These results are consistent with the idea that the neutron stars have been spun up by accreting matter form their companion stars.