Discovery of a nebula around PSR1957+20

Abstract

The eclipsing binary pular PSR1957 + 20 is presumed to have been spun up to its present rotation period of 1.6 ms by an earlier episode of accretion. We report here the discovery of an Hα emission nebula around PSR1957 + 20, the first nebula to be found around a 'recycled', and thus ancient, pulsar. We suggest that the Hα emission arises in shocks driven into the interstellar medium by a relativistic wind from the pulsar. The nebular spectrum shows similarities to fast shocks in young supernova remnants like Tycho; the absence of the usual optical forbidden lines is explained by models of the excitation of neutral atoms drifting into the post-shock region, but the large observed Balmer decrement (I(Hα)/I(Hβ)≳12) is more mysterious. Between 1% and 10% of the rotational luminosity is absorbed in the region outlined by the Hα emission, with the bulk of the relativistic wind flowing away from the pulsar. This nebula adds to a growing number of pulsar wind nebulae, like the one around PSR1951 + 32 in CTB80 (ref. 3).