The study of the younger, and brighter, pulsars is important to understand
the optical emission properties of isolated neutron stars. PSRB0540-69, the
second brightest (V~22) optical pulsar, is obviously a very interesting target
for these investigations. The aim of this work is threefold: constraining the
pulsar proper motion and its velocity on the plane of the sky through optical
astrometry, obtaining a more precise characterisation of the pulsar optical
spectral energy distribution (SED) through a consistent set of multi-band,
high-resolution, imaging photometry observations, measuring the pulsar optical
phase-averaged linear polarisation, for which only a preliminary and uncertain
measurement was obtained so far from ground-based observations. We performed
high-resolution observations of PSRB0540-69 with the WFPC2 aboard the HST, in
both direct imaging and polarimetry modes. From multi-epoch astrometry we set a
3sigma upper limit of 1 mas/yr on the pulsar proper motion, implying a
transverse velocity <250 km/s at the 50 kpc LMC distance. Moreover, we
determined the pulsar absolute position with an unprecedented accuracy of 70
mas. From multi-band photometry we characterised the pulsar power-law spectrum
and we derived the most accurate measurement of the spectral index
(0.70+/-0.07) which indicates a spectral turnover between the optical and X-ray
bands. Finally, from polarimetry we obtained a new measurement of the pulsar
phase-averaged polarisation degree (16+/-4%),consistent with magnetosphere
models depending on the actual intrinsic polarisation degree and depolarisation
factor, and we found that the polarisation vector (22+/-12deg position angle)
is possibly aligned with the semi-major axis of the pulsar-wind nebula and with
the apparent proper motion direction of its bright emission knot.Comment: 14 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic