2009AJ....137.3509N

We present optical spectropolarimetric observations of three microquasars, Cyg X-1, LS 5039, and LS I +61° 303. Polarimetry can bring us geometrical information on the innermost part of the circumstellar media. For Cyg X-1, the observed continuum polarization shows sinusoidal time variation (Δ ∼ 0.2%) correlated with the orbital phase, as indicated by past studies. The constant component of the polarization has significantly changed (Δ ∼ 0.7%) within a few decades, indicating variation of scattering material surrounding the binary. There is no apparent change of polarization across the Hα emission line in Cyg X-1. For LS 5039, the observed polarization shows no significant orbital variation. The wavelength dependence is almost consistent with the typical interstellar polarization (ISP) and we conclude that the ISP is a dominant component of the observed polarization. For LS I +61° 303, the observed polarization is roughly consistent with our previous paper, except for a slight change in the intrinsic polarization and also in the equivalent width of the Hα emission line. These imply a long-term evolution of the Be disk around the mass donor. The position angle of intrinsic polarization at ≃25° was constant from 2005 through 2008. Our observations suggest that the optical polarization intrinsic to a microquasar with an high mass donor can be explained by the light scattering in the circumstellar matter around the mass donor. A polarization signal from the vicinity of the compact star (including jet and accretion disk) seems absent.