New limits on the population of millisecond pulsars in the galactic plane

This paper investigates the disc population of millisecond pulsars using the results of the recent high-frequency survey of the southern galactic plane by Johnston et al. and the corresponding earlier survey of the northern galactic plane by Clifton & Lyne. It is shown that the number of millisecond pulsars in the Galaxy with luminosities similar to PSR 1937 + 21 and PSR 1855 + 09 need not exceed ∼ 60 and ∼ 2500 respectively. These limits are much lower than previous estimates, partly because of an improved distance estimate to 1855 + 09. If all millisecond pulsars have similar luminosities to that of 1855 + 09, then it is possible that all millisecond pulsars evolve from low-mass X-ray binaries. However, only poor constraints can be placed on the populations of very steep spectrum objects like PSR 1957 + 20, and intrinsically low-luminosity pulsars such as PSR 1257 + 12, which could be of order 105 in number. A low-frequency survey of the southern hemisphere for millisecond pulsars now under way should be able to place strong limits on the galactic population and their birthrates.