Optical and infrared polarimetry of the transient LMXB Centaurus X-4 in quiescence

Aims. We present the first optical and infrared polarimetric study of the low-mass transient X-ray binary Centaurus X-4 during its quiescent phase. This work aims to search for an intrinsic linear polarisation component in the system emitted radiation that might be due to, e. g., synchrotron emission from a compact jet, or to Thomson scattering with free electrons in an accretion disc. Methods. Multiband (BVRI) optical polarimetric observations were obtained over two nights in 2008 at the ESO La Silla 3.6 m telescope, equipped with the EFOSC2 instrument used in polarimetric mode. These observations cover about the 30% of the 15.1 h orbital period. We obtained J-band observations in 2007 with the NICS instrument on the Telescopio Nazionale Galileo (TNG) at La Palma, for 1 h total of observations. Results. We obtained 3 sigma upper limits to the polarisation degree in all of the optical bands, with the most constraining limit being in the I-band, where P-I < 0.5%. We have noticed no significant phase-correlated variability in any of the filters. The J-band observations provided a 6% upper limit on the polarisation level. Conclusions. The constraining upper limits on the polarisation in the optical (above all in the I-band), allowed us to estimate the contribution of the possible emission of a relativistic particles jet to the total system radiation to be less than 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, without any significant additional component in the infrared. Because of the low S/N it was not possible to investigate the possible dependency of the polarisation degree from the wavelength, which 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 are required to examine this hypothesis in depth, searching for significant phase-correlated variability.