Coherent radiation patterns suggested by single-pulse observations of a millisecond pulsar

We have observed intense radiation spikes from the millisecond pulsar PSR J0437-4715 and find their distribution in pulsar spin phase to be quite unlike the integrated pulse profile. The spikes are observed almost exclusively in a 10 degrees phase window centered on the main pulse. Within that window the phase distribution has a periodic variation, producing a pattern reminiscent of the diffraction fringes associated with coherent emission from a finite aperture. We propose a source model in which the spikes are emitted coherently from a region with spatial scale approximate to 100 m.