THE POLARIZATION OF X-RAY BINARIES IN THE ULTRAVIOLET .1. CYGNUS-XR-1, 4U-0900-40, AND 4U-1700-37

Cyg XR-1, 4U 0900-40, and 4U 1700-37 were monitored polarimetrically a nd photometrically in the ultraviolet (UV) with the High Speed Photome ter (HSP) on the Hubble Space Telescope (HST) at 6-10 different phases during a single binary orbit. Polarimetric observations were obtained in a bandpass centered at 2770 Angstrom; 4U 0900-40 was also monitore d polarimetrically in a bandpass centered at 3270 Angstrom. Photometri c observations were obtained in a bandpass centered at 2480 Angstrom. The UV polarization is variable in all three systems, consistent with a previously discovered intrinsic component of polarization attributed to scattering of light from the supergiant primary by a gas stream be tween the primary and the neutron star (4U 0900-40 and 4U 1700-37) or black hole (Cyg XR-1) secondary. The amplitude of the polarimetric var iability is much larger in the UV than in the visible, confirming a pr ediction of the model that identifies Rayleigh scattering as the princ ipal mechanism producing the intrinsic polarization. The polarimetric variability as a function of phase fits the standard scattering model of Brown, McLean, & Emslie only if an additional component of polariza tion occurs near first quadrature in the two systems with neutron star secondaries.