Infrared polarimetry of the microquasars H1743-322, XTE J1550-564 and GRO J1655-40

Aims. The optical/IR emission from X-ray binaries can be intrinsically polarised because of light scattered within the system or because there is significant synchrotron emission at high frequencies from a compact jet. We report on polarised light observations of three microquasars (X-ray binaries with radio jets). Methods. K_s band polarimetric observations were obtained at the ESO NTT using SOFI. The linear polarisation of the three systems is derived relative to that of other stars in the field-of-view, so as to distinguish intrinsic polarisation from interstellar polarisation. Results. H1743-322 was observed in outburst in 2003 and 2004. We measure a linear polarisation of 2.25 ± 0.75% comparable to that of other nearby stars. The polarisation is very likely interstellar, consistent with the large extinction (A_v ≈ 11) along the line-of-sight. GRO J1655-40 was observed in quiescence and shows no evidence for intrinsic polarisation. XTE J1550-564 was observed during a weak X-ray outburst. It shows a higher polarisation (at the 2.5σ level) than neighbouring stars within 1′. Further studies are needed to establish the nature of this possible intrinsic polarisation.