  We present the first optical and infrared polarimetric study of the low mass
transient X-ray binary Cen X-4 during its quiescent phase. This work is aimed
to search for an intrinsic linear polarisation component in the system emitted
radiation that might be due, e.g., to synchrotron emission from a compact jet,
or to Thomson scattering with free electrons in an accretion disc. Multiband
(BVRI) optical polarimetric observations were obtained during two nights in
2008 at the ESO La Silla 3.6 m telescope (EFOSC2) in polarimetric mode. These
observations cover about the 30% of the 15.1 hours orbital period. J-band
observations were obtained in 2007 with the NICS (TNG) instrument at La Palma,
for a totality of 1 hour observation. We obtained 3-sigma upper limits to the
polarisation degree in all the optical bands, with the most constraining one
being in the I-band (P<0.5%). No phase-correlated variability has been noticed
in all the filters. The J-band observations provided a 6% upper limit on the
polarisation level. The constraining upper limits to the polarisation in the
optical allowed us to evaluate the contribution of the possible emission of a
relativistic particles jet to the total system radiation to be less then the
10%. This is in agreement with the observation of a spectral energy
distribution typical of a single black body of a K-spectral type main sequence
star irradiated from the compact object. Due to the low S/N ratio it was not
possible to investigate the possible dependency of the polarisation degree from
the wavelength, that could be suggestive of polarisation induced by Thomson
scattering of radiation with free electrons in the outer part of the accretion
disc. Observations with higher S/N ratio are required to examine in depth this
hypothesis, searching for significant phase-correlated variability.
