Discovery of a nebula around PSR1957+20
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).
© 1988 Nature Publishing Group. Received 30 August; accepted 22 September 1988. We are grateful for stimulating discussions with E. S. Phinney and C. R. Evans, which led to substantial improvements of the paper. S.R.K. is supported partly by a US Presidential Young Investigator's award and the Sloan Foundation. J.J .H. is supported by Caltech's Infrared Processing and Analysis Center.