A Radio Survey for Pulsar Wind Nebulae

Abstract

We have imaged the fields of 35 pulsars with the VLA 1 at 8.4 GHz tosearch for evidence that the pulsars have deposited energy into theirsurroundings in the form of an extended wind nebula. Candidates haveboth a wide range of transverse velocities and rotational energylosses. Of the 35 pulsars that we imaged, no pointlike or extendedemission was detected toward 21 sources, to a typical rms surfacebrightness sensitivity of 40 mu Jy beam-1, for a beam size of 0."8.Pointlike emission was detected toward 14 pulsars with peak fluxdensities between 0.2 and 3 mJy. After a careful examination of eachsource we conclude that these detections are simply the time-averagedpulsed flux density from the pulsars themselves, and thus no newpulsar wind nebulae were discovered. From the surface brightnesslimits and an estimate of the size of the region where the pulsarwind is first shocked, we deduce that the fraction of the pulsars'spindown energy going into producing a compact radio nebula must beless than 10-6. In contrast, this fraction is 2 orders of magnitudelarger for the six pulsars which are known unambiguously to havea radio pulsar wind nebula around them. Several explanations aregiven for this behavior, and we conclude that only young pulsars withhigh rotational energy losses produce an observable radio nebula.