An Unusual Pulsar Wind Nebula Associated with PSR B0906−49

We report on Australia Telescope Compact Array observations of the ~10^5 yrold pulsar PSR B0906-49. In an image containing only off-pulse emission, wefind a weak, slightly extended source coincident with the pulsar's position,which we argue is best interpreted as a pulsar wind nebula (PWN). A trail ofemission extending behind the pulsar aligns with the major axis of the PWN, andimplies that the pulsar is moving north-west with projected velocity ~60 km/s,consistent with its scintillation speed. The consequent density we infer forthe pulsar's environment is >2 cm^(-3), so that the PWN around PSR B0906-49 isconfined mainly by the high density of its surroundings rather than by thepulsar's velocity. Other properties of the system such as the PWN's lowluminosity and apparent steep spectrum, and the pulsar's large characteristicage, lead us to suggest that this nebula is substantially different from otherradio PWNe, and may represent a transition between young pulsars with prominentradio PWNe and older pulsars for which no radio PWN has been detected. Werecommend that further searches for radio PWNe should be made as here: at lowfrequencies and with the pulsed emission subtracted.