A Very Luminous Binary Millisecond Pulsar

We report the discovery of a field binary millisecond pulsar, J0218+4232, with a period of 2.3 ms and in a 2.0 day binary orbit with a greater than or similar to 0.16 M. companion. The new pulsar was serendipitously discovered as a steep-spectrum, highly polarized, compact radio source during imaging observations at Westerbork, and was later confirmed to be a pulsar with observations carried out with the 76 m antenna at Jodrell Bank. With a dispersion measure of 61 pc cm(-3), it lies outside the electron layer in the direction I = 140 degrees, b = -18 degrees. At a distance of more than 5.7 kpc, it is the farthest known field millisecond pulsar and has a radio luminosity L(400) comparable to that of PSR B1937+21. It appears that a significant fraction of the radio emission is not pulsed. This, together with the extremely broad pulse profile, suggests that we are looking at an aligned rotator.