The optical polarisation of the Vela pulsar revisited

Aims. In this work we present a revised measurement of the phase-averaged optical polarisation of the Vela pulsar ( PSR B0833-45), for which only one value has been published so far ( Wagner & Seifert 2000, ASP Conf. Ser., 202, 315). Methods. Our measurement has been obtained through an accurate reanalysis of archival polarisation observations with the FORS1 instrument at the VLT. Results. We measured a phase-averaged linear polarisation degree P-L = 9.4% +/- 4% and a position angle theta = 146 degrees +/- 11 degrees, very close to the ones for the axis of symmetry of the X-ray arcs and jets detected by Chandra and of the pulsar proper motion. Conclusions. We compared the measured phase-averaged optical polarisation with the expectations of different pulsars' magnetosphere models. We find that all models consistently predict values of the phase-averaged linear polarisation that are too high with respect to the observed one. This is probably a consequence of present models' limitations that neglect the contributions of various depolarisation effects. Interestingly, for the outer gap model we find that, when assuming synchrotron radiation for the optical emission, the observed polarisation position angle also implies an alignment between the pulsar rotational axis and the axis of symmetry of the X-ray arcs and jets.