Polarimetric Survey of Low-Mass X-ray Binaries

In many low-mass X-ray binaries the optical light is dominated by an accretion disk. The bulk of the accretion disk is ionized, and in many models the surface layers are threaded by a magnetic field. Polarimetric observations can give information on the scattering properties of the disk surface. Polarimetric observations of three low-mass X-ray binaries, Sco X-1, Her X-1 and XTE J 1118+480, in optical wavelenghts are presented. A strong interstellar polarization component dominates the polarization of Sco X-1, but we also detected its weak intrinsic linear polarization. Marginally variable polarization is possibly seen in XTE J 1118+480. The circular polarization of Sco X-1 and XTE J1118+480 is consistent with zero. Contrary to previous claims, the linear polarization of Her X-1 seems to be constant. The observed polarization can be explained with a combination of interstellar polarization and electron scattering in the ionized accretion disk. We also derive the upper limit for the large-scale ordered magnetic field in the accretion disk. Some linear and circular polarimetry for the high-mass X-ray binary SS 433 is also presented.