Discovery of four isolated millisecond pulsars

We report the discovery of four isolated millisecond pulsars found as part of the Parkes 436 MHz survey of the southern sky. Three of the pulsars, PSRs J1024-0719, J1744-1134, and J2124-3358, are close to the Sun (d < 360 pc) and have very low luminosities, less than or similar to 0.5 mJy kpc(2). The other, PSR J0711-6830, is of intermediate luminosity. The four least luminous millisecond pulsars presently known are all isolated objects, even though more than 75% of the known disk millisecond pulsars are binary. A Kolmogorov-Smirnov analysis confirms that the luminosity distributions of the binary and isolated millisecond pulsars are different at the 99.5% confidence level. We can find no simple explanation for this fact. The low-luminosity millisecond pulsars reported here exacerbate the birthrate discrepancy with their assumed progenitors, the low-mass X-ray binaries. None of the pulsars exhibits any evidence of a planetary system such as that observed around PSR B1257+12, indicating that planetary formation around millisecond pulsars is rare.