2006PASJ...58.1015N

We present an optical linear spectropolarimetry study of the microquasar LS I +61°303. The continuum emission is mildly polarized (up to 1.3%) and shows almost no temporal change. We found a distinct change of polarization across the Hα emission line, indicating the existence of a polarization component intrinsic to the microquasar. We estimated the interstellar polarization component from polarization of the Hα line and derived the intrinsic polarization component. The wavelength dependence of the intrinsic component is well explained by Thomson scattering in an equatorial disk of a Be-type mass donor. The position angle (PA) of the intrinsic polarization is ∼25°, and represents the rotational axis of the Be disk. This PA is nearly perpendicular to the PA of a radio jet found during quiescent phases. Assuming an orthogonal disk-jet geometry around the compact star, the rotational axis of the accretion disk is almost perpendicular to that of the Be disk. Moreover, according to the orbital parameters of the microquasar, the compact star is likely to get across the Be disk around their periastron passage. We discuss the peculiar circumstellar structure of this microquasar inferred based on our observation, and suggest a possible connection with its high-energy activities.