HST optical polarimetry of the Vela pulsar and nebula

Polarization measurements of pulsars offer a unique insight into the geometry of the emission regions in the neutron star magnetosphere. Therefore, they provide observational constraints on the different models proposed for the pulsar emission mechanisms. Optical polarization data of the Vela pulsar was obtained from the Hubble Space Telescope (HST) archive. The data, obtained in two filters (F606W, central wavelength = 590.70 nm and F550M, central wavelength = 558.15 nm), consist of a series of observations of the pulsar taken with the HST/Advanced Camera for Surveys and cover a time span of 5 d. These data have been used to carry out the first high spatial resolution and multi-epoch study of the polarization of the pulsar. We produced polarization vector maps of the region surrounding the pulsar and measured the degree of linear polarization (P. D.) and the position angle (P.A.) of the pulsar's integrated pulse beam. We obtained P. D. = 8.1 +/- 0.7 per cent and P.A. = 146.degrees 3 +/- 2.degrees 4, averaged over the time span covered by these observations. These results not only confirm those originally obtained by Wagner & Seifert and Mignani et al., both using the Very Large Telescope, but are of greater precision. Furthermore, we confirm that the P. A. of the pulsar polarization vector is aligned with the direction of the pulsar proper motion. The pulsar wind nebula is undetected in polarized light as is the case in unpolarized light, down to a flux limit of 26.8 mag arcsec(-2).