Limits on radio emission from pulsar wind nebulae

We report on a sensitive survey for radio pulsar wind nebulae (PWN) towards 27 energetic and/or high-velocity pulsars. Observations were carried out at 1.4 GHz using the Very Large Array and the Australia Telescope Compact Array and utilized pulsar-gating to search for off-pulse emission. These observing parameters resulted in a considerably more sensitive search than previous surveys and could detect PWN over a much wider range of spatial scales (and hence ambient densities and pulsar velocities). However, no emission clearly corresponding to a PWN was discovered. Based on these non-detections we argue that the young and energetic pulsars in our sample have winds which are typical of young pulsars, but produce unobservable PWN because they reside in low-density (n similar to 0.003 cm(-3)) regions of the interstellar medium. However, non-detection of PWN around older and less energetic pulsars can only be explained if the radio luminosity of their winds is less than 10(-5) of their spin-down luminosity, implying an efficiency at least an order of magnitude smaller than that seen for young pulsars.